diff --git a/Containers/UserSettings/EmailSettings/EmailSettings.tsx b/Containers/UserSettings/EmailSettings/EmailSettings.tsx
index e960a0b..86f6e87 100644
--- a/Containers/UserSettings/EmailSettings/EmailSettings.tsx
+++ b/Containers/UserSettings/EmailSettings/EmailSettings.tsx
@@ -10,16 +10,9 @@ import { SpinnerDotted } from 'spinners-react';
 import Error from '~/Components/UI/Error/Error';
 import Info from '~/Components/UI/Info/Info';
 import { useFormStatus } from '~/hooks/useFormStatus';
+import { EmailSettingDTO } from '~/types/api/settings.types';
 
-type EmailDataForm = {
-  email: string;
-};
-
-type EmailSettingsProps = {
-  email?: string;
-};
-
-export default function EmailSettings(props: EmailSettingsProps) {
+export default function EmailSettings(props: EmailSettingDTO) {
   //Var
   const toastOptions: ToastOptions = {
     position: 'top-right',
@@ -36,7 +29,7 @@ export default function EmailSettings(props: EmailSettingsProps) {
     handleSubmit,
     reset,
     formState: { errors, isSubmitting, isValid },
-  } = useForm<EmailDataForm>({ mode: 'onChange' });
+  } = useForm<EmailSettingDTO>({ mode: 'onChange' });
 
   const { isLoading, error, setIsLoading, handleError, clearError } = useFormStatus();
 
@@ -44,7 +37,7 @@ export default function EmailSettings(props: EmailSettingsProps) {
   const [info, setInfo] = useState(false);
 
   ////Functions
-  const formSubmitHandler = async (data: EmailDataForm) => {
+  const formSubmitHandler = async (data: EmailSettingDTO) => {
     clearError();
     setIsLoading(true);
 
diff --git a/Containers/UserSettings/PasswordSettings/PasswordSettings.tsx b/Containers/UserSettings/PasswordSettings/PasswordSettings.tsx
index a05cf8e..980ea47 100644
--- a/Containers/UserSettings/PasswordSettings/PasswordSettings.tsx
+++ b/Containers/UserSettings/PasswordSettings/PasswordSettings.tsx
@@ -8,11 +8,7 @@ import { SpinnerDotted } from 'spinners-react';
 //Components
 import Error from '~/Components/UI/Error/Error';
 import { useFormStatus } from '~/hooks/useFormStatus';
-
-type PasswordDataForm = {
-  oldPassword: string;
-  newPassword: string;
-};
+import { PasswordSettingDTO } from '~/types/api/settings.types';
 
 export default function PasswordSettings() {
   //Var
@@ -31,12 +27,12 @@ export default function PasswordSettings() {
     handleSubmit,
     reset,
     formState: { errors, isSubmitting, isValid },
-  } = useForm<PasswordDataForm>({ mode: 'onChange' });
+  } = useForm<PasswordSettingDTO>({ mode: 'onChange' });
 
   const { isLoading, error, setIsLoading, handleError, clearError } = useFormStatus();
 
   ////Functions
-  const formSubmitHandler = async (data: PasswordDataForm) => {
+  const formSubmitHandler = async (data: PasswordSettingDTO) => {
     clearError();
     setIsLoading(true);
 
diff --git a/Containers/UserSettings/UsernameSettings/UsernameSettings.tsx b/Containers/UserSettings/UsernameSettings/UsernameSettings.tsx
index 9766a8a..3f82539 100644
--- a/Containers/UserSettings/UsernameSettings/UsernameSettings.tsx
+++ b/Containers/UserSettings/UsernameSettings/UsernameSettings.tsx
@@ -10,16 +10,9 @@ import { SpinnerDotted } from 'spinners-react';
 import Error from '~/Components/UI/Error/Error';
 import Info from '~/Components/UI/Info/Info';
 import { useFormStatus } from '~/hooks/useFormStatus';
+import { UsernameSettingDTO } from '~/types/api/settings.types';
 
-type UsernameDataForm = {
-  username: string;
-};
-
-type UsernameSettingsProps = {
-  username?: string;
-};
-
-export default function UsernameSettings(props: UsernameSettingsProps) {
+export default function UsernameSettings(props: UsernameSettingDTO) {
   //Var
   const toastOptions: ToastOptions = {
     position: 'top-right',
@@ -36,7 +29,7 @@ export default function UsernameSettings(props: UsernameSettingsProps) {
     handleSubmit,
     reset,
     formState: { errors, isSubmitting, isValid },
-  } = useForm<UsernameDataForm>({ mode: 'onChange' });
+  } = useForm<UsernameSettingDTO>({ mode: 'onChange' });
 
   const { isLoading, error, setIsLoading, handleError, clearError } = useFormStatus();
 
@@ -44,7 +37,7 @@ export default function UsernameSettings(props: UsernameSettingsProps) {
   const [info, setInfo] = useState(false);
 
   ////Functions
-  const formSubmitHandler = async (data: UsernameDataForm) => {
+  const formSubmitHandler = async (data: UsernameSettingDTO) => {
     clearError();
     setIsLoading(true);
 
diff --git a/helpers/functions/index.ts b/helpers/functions/index.ts
new file mode 100644
index 0000000..94dff3a
--- /dev/null
+++ b/helpers/functions/index.ts
@@ -0,0 +1,8 @@
+export * from './auth';
+export * from './fileHelpers';
+export * from './isSshPubKeyDuplicate';
+export * from './lanCommandOption';
+export * from './nodemailerSMTP';
+export * from './repoHistory';
+export * from './timestampConverter';
+export * from './tokenController';
diff --git a/pages/api/account/updateAppriseMode.ts b/pages/api/account/updateAppriseMode.ts
index 6633aba..2e6b15b 100644
--- a/pages/api/account/updateAppriseMode.ts
+++ b/pages/api/account/updateAppriseMode.ts
@@ -1,4 +1,3 @@
-// Imports
 import { getUsersList, updateUsersList } from '~/helpers/functions/fileHelpers';
 import { authOptions } from '../auth/[...nextauth]';
 import { getServerSession } from 'next-auth/next';
diff --git a/pages/api/account/updateEmail.js b/pages/api/account/updateEmail.js
deleted file mode 100644
index 29a4bc5..0000000
--- a/pages/api/account/updateEmail.js
+++ /dev/null
@@ -1,84 +0,0 @@
-//Lib
-import { promises as fs } from 'fs';
-import path from 'path';
-import { authOptions } from '../auth/[...nextauth]';
-import { getServerSession } from 'next-auth/next';
-
-export default async function handler(req, res) {
-  if (req.method == 'PUT') {
-    //Verify that the user is logged in.
-    const session = await getServerSession(req, res, authOptions);
-    if (!session) {
-      res.status(401).json({ message: 'You must be logged in.' });
-      return;
-    }
-
-    //The data we expect to receive
-    let { email } = req.body;
-
-    //Read the users file
-    //Find the absolute path of the json directory
-    const jsonDirectory = path.join(process.cwd(), '/config');
-    let usersList = await fs.readFile(jsonDirectory + '/users.json', 'utf8');
-    //Parse the usersList
-    usersList = JSON.parse(usersList);
-
-    //1 : We check that we receive data.
-    if (!email) {
-      //If a variable is empty.
-      res.status(400).json({ message: 'A field is missing.' });
-      return;
-    }
-
-    //2 : control the data
-    const emailRegex = new RegExp(
-      /^(([^<>()\[\]\\.,;:\s@"]+(\.[^<>()\[\]\\.,;:\s@"]+)*)|(".+"))@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$/
-    );
-    if (!emailRegex.test(email)) {
-      res.status(400).json({ message: 'Your email is not valid' });
-      return;
-    }
-
-    //3 : Verify that the user of the session exists
-    const userIndex = usersList.map((user) => user.username).indexOf(session.user.name);
-    if (userIndex === -1) {
-      res.status(400).json({
-        message: 'User is incorrect. Please, logout to update your session.',
-      });
-      return;
-    }
-
-    //4 : Change the email
-    try {
-      //Modify the email for the user
-      let newUsersList = usersList.map((user) =>
-        user.username == session.user.name ? { ...user, email: email } : user
-      );
-      //Stringify the new users list
-      newUsersList = JSON.stringify(newUsersList);
-      //Write the new JSON
-      await fs.writeFile(jsonDirectory + '/users.json', newUsersList, (err) => {
-        if (err) console.log(err);
-      });
-      res.status(200).json({ message: 'Successful API send' });
-    } catch (error) {
-      //Log for backend
-      console.log(error);
-      //Log for frontend
-      if (error.code == 'ENOENT') {
-        res.status(500).json({
-          status: 500,
-          message: 'No such file or directory',
-        });
-      } else {
-        res.status(500).json({
-          status: 500,
-          message: 'API error, contact the administrator',
-        });
-      }
-      return;
-    }
-  } else {
-    res.status(405).json({ message: 'Bad request on API' });
-  }
-}
diff --git a/pages/api/account/updateEmail.ts b/pages/api/account/updateEmail.ts
new file mode 100644
index 0000000..5614437
--- /dev/null
+++ b/pages/api/account/updateEmail.ts
@@ -0,0 +1,57 @@
+import { getUsersList, updateUsersList } from '~/helpers/functions/fileHelpers';
+import { authOptions } from '../auth/[...nextauth]';
+import { getServerSession } from 'next-auth/next';
+import { NextApiRequest, NextApiResponse } from 'next';
+import { EmailSettingDTO } from '~/types/api/settings.types';
+import { ErrorResponse } from '~/types/api/error.types';
+
+export default async function handler(
+  req: NextApiRequest & { body: EmailSettingDTO },
+  res: NextApiResponse<ErrorResponse>
+) {
+  if (req.method !== 'PUT') {
+    return res.status(405);
+  }
+
+  const session = await getServerSession(req, res, authOptions);
+  if (!session) {
+    return res.status(401);
+  }
+
+  const { email } = req.body;
+
+  if (!email) {
+    return res.status(422).json({ message: 'Unexpected data' });
+  }
+
+  try {
+    const usersList = await getUsersList();
+    const userIndex = usersList.findIndex((user) => user.username === session.user?.name);
+
+    if (userIndex === -1) {
+      return res.status(400).json({
+        message: 'User is incorrect. Please, logout to update your session.',
+      });
+    }
+
+    if (usersList.some((user) => user.email === email)) {
+      return res.status(400).json({ message: 'Email already exists' });
+    }
+
+    const updatedUsersList = usersList.map((user, index) =>
+      index === userIndex ? { ...user, email } : user
+    );
+
+    await updateUsersList(updatedUsersList);
+    return res.status(200).json({ message: 'Successful API send' });
+  } catch (error: any) {
+    console.error(error);
+    return res.status(500).json({
+      status: 500,
+      message:
+        error.code === 'ENOENT'
+          ? 'No such file or directory'
+          : 'API error, contact the administrator',
+    });
+  }
+}
diff --git a/pages/api/account/updateEmailAlert.js b/pages/api/account/updateEmailAlert.js
deleted file mode 100644
index 9a97c99..0000000
--- a/pages/api/account/updateEmailAlert.js
+++ /dev/null
@@ -1,74 +0,0 @@
-//Lib
-import { promises as fs } from 'fs';
-import path from 'path';
-import { authOptions } from '../auth/[...nextauth]';
-import { getServerSession } from 'next-auth/next';
-
-export default async function handler(req, res) {
-  if (req.method == 'PUT') {
-    //Verify that the user is logged in.
-    const session = await getServerSession(req, res, authOptions);
-    if (!session) {
-      res.status(401).json({ message: 'You must be logged in.' });
-      return;
-    }
-
-    //The data we expect to receive
-    let { emailAlert } = req.body;
-
-    //Read the users file
-    //Find the absolute path of the json directory
-    const jsonDirectory = path.join(process.cwd(), '/config');
-    let usersList = await fs.readFile(jsonDirectory + '/users.json', 'utf8');
-    //Parse the usersList
-    usersList = JSON.parse(usersList);
-
-    //1 : control the data
-    if (typeof emailAlert != 'boolean') {
-      res.status(422).json({ message: 'Unexpected data' });
-      return;
-    }
-
-    //2 : Verify that the user of the session exists
-    const userIndex = usersList.map((user) => user.username).indexOf(session.user.name);
-    if (userIndex === -1) {
-      res.status(400).json({
-        message: 'User is incorrect. Please, logout to update your session.',
-      });
-      return;
-    }
-
-    //3 : Change the emailAlert settings
-    try {
-      //Modify the email for the user
-      let newUsersList = usersList.map((user) =>
-        user.username == session.user.name ? { ...user, emailAlert: emailAlert } : user
-      );
-      //Stringify the new users list
-      newUsersList = JSON.stringify(newUsersList);
-      //Write the new JSON
-      await fs.writeFile(jsonDirectory + '/users.json', newUsersList, (err) => {
-        if (err) console.log(err);
-      });
-      res.status(200).json({ message: 'Successful API send' });
-    } catch (error) {
-      //Log for backend
-      console.log(error);
-      //Log for frontend
-      if (error.code == 'ENOENT') {
-        res.status(500).json({
-          status: 500,
-          message: 'No such file or directory',
-        });
-      } else {
-        res.status(500).json({
-          status: 500,
-          message: 'API error, contact the administrator',
-        });
-      }
-      return;
-    }
-  } else {
-    res.status(405).json({ message: 'Bad request on API' });
-  }
-}
diff --git a/pages/api/account/updateEmailAlert.ts b/pages/api/account/updateEmailAlert.ts
new file mode 100644
index 0000000..116ee46
--- /dev/null
+++ b/pages/api/account/updateEmailAlert.ts
@@ -0,0 +1,53 @@
+import { getUsersList, updateUsersList } from '~/helpers/functions/fileHelpers';
+import { authOptions } from '../auth/[...nextauth]';
+import { getServerSession } from 'next-auth/next';
+import { EmailAlertDTO } from '~/types/api/notifications.types';
+import { NextApiRequest, NextApiResponse } from 'next';
+import { ErrorResponse } from '~/types/api/error.types';
+
+export default async function handler(
+  req: NextApiRequest & { body: EmailAlertDTO },
+  res: NextApiResponse<ErrorResponse>
+) {
+  if (req.method !== 'PUT') {
+    return res.status(405);
+  }
+
+  const session = await getServerSession(req, res, authOptions);
+  if (!session) {
+    return res.status(401);
+  }
+
+  const { emailAlert } = req.body;
+
+  if (typeof emailAlert !== 'boolean') {
+    return res.status(422).json({ message: 'Unexpected data' });
+  }
+
+  try {
+    const usersList = await getUsersList();
+    const userIndex = usersList.findIndex((u) => u.username === session.user?.name);
+
+    if (userIndex === -1) {
+      return res
+        .status(400)
+        .json({ message: 'User is incorrect. Please, logout to update your session.' });
+    }
+
+    const updatedUsersList = usersList.map((user, index) =>
+      index === userIndex ? { ...user, emailAlert } : user
+    );
+
+    await updateUsersList(updatedUsersList);
+    return res.status(200).json({ message: 'Successful API send' });
+  } catch (error: any) {
+    console.error(error);
+    return res.status(500).json({
+      status: 500,
+      message:
+        error.code === 'ENOENT'
+          ? 'No such file or directory'
+          : 'API error, contact the administrator',
+    });
+  }
+}
diff --git a/pages/api/account/updatePassword.js b/pages/api/account/updatePassword.js
deleted file mode 100644
index 26582f1..0000000
--- a/pages/api/account/updatePassword.js
+++ /dev/null
@@ -1,84 +0,0 @@
-//Lib
-import { hashPassword, verifyPassword } from '../../../helpers/functions/auth';
-import { promises as fs } from 'fs';
-import path from 'path';
-import { authOptions } from '../auth/[...nextauth]';
-import { getServerSession } from 'next-auth/next';
-
-export default async function handler(req, res) {
-  if (req.method == 'PUT') {
-    //Verify that the user is logged in.
-    const session = await getServerSession(req, res, authOptions);
-    if (!session) {
-      res.status(401).json({ message: 'You must be logged in.' });
-      return;
-    }
-
-    //The data we expect to receive
-    let { oldPassword, newPassword } = req.body;
-
-    //Read the users file
-    //Find the absolute path of the json directory
-    const jsonDirectory = path.join(process.cwd(), '/config');
-    let usersList = await fs.readFile(jsonDirectory + '/users.json', 'utf8');
-    //Parse the usersList
-    usersList = JSON.parse(usersList);
-
-    //1 : We check that we receive data for each variable.
-    if (!oldPassword || !newPassword) {
-      //If a variable is empty.
-      res.status(400).json({ message: 'A field is missing.' });
-      return;
-    }
-    //Hash the new password
-    newPassword = await hashPassword(newPassword);
-
-    //2 : Verify that the user of the session exists
-    const userIndex = usersList.map((user) => user.username).indexOf(session.user.name);
-    if (userIndex === -1) {
-      res.status(400).json({ message: 'User is incorrect.' });
-      return;
-    }
-    const user = usersList[userIndex];
-
-    //3 : Check that the old password is correct
-    const isValid = await verifyPassword(oldPassword, user.password);
-    if (!isValid) {
-      res.status(400).json({ message: 'Old password is incorrect.' });
-      return;
-    }
-
-    //4 : Change the password
-    try {
-      //Modify the password for the user
-      let newUsersList = usersList.map((user) =>
-        user.username == session.user.name ? { ...user, password: newPassword } : user
-      );
-      //Stringify the new users list
-      newUsersList = JSON.stringify(newUsersList);
-      //Write the new JSON
-      await fs.writeFile(jsonDirectory + '/users.json', newUsersList, (err) => {
-        if (err) console.log(err);
-      });
-      res.status(200).json({ message: 'Successful API send' });
-    } catch (error) {
-      //Log for backend
-      console.log(error);
-      //Log for frontend
-      if (error.code == 'ENOENT') {
-        res.status(500).json({
-          status: 500,
-          message: 'No such file or directory',
-        });
-      } else {
-        res.status(500).json({
-          status: 500,
-          message: 'API error, contact the administrator',
-        });
-      }
-      return;
-    }
-  } else {
-    res.status(405).json({ message: 'Bad request on API' });
-  }
-}
diff --git a/pages/api/account/updatePassword.ts b/pages/api/account/updatePassword.ts
new file mode 100644
index 0000000..d0dac57
--- /dev/null
+++ b/pages/api/account/updatePassword.ts
@@ -0,0 +1,61 @@
+import { getUsersList, updateUsersList, hashPassword, verifyPassword } from '~/helpers/functions';
+import { authOptions } from '../auth/[...nextauth]';
+import { getServerSession } from 'next-auth/next';
+import { NextApiRequest, NextApiResponse } from 'next';
+import { ErrorResponse } from '~/types/api/error.types';
+import { PasswordSettingDTO } from '~/types/api/settings.types';
+
+export default async function handler(
+  req: NextApiRequest & { body: PasswordSettingDTO },
+  res: NextApiResponse<ErrorResponse>
+) {
+  if (req.method !== 'PUT') {
+    return res.status(405);
+  }
+
+  const session = await getServerSession(req, res, authOptions);
+  if (!session) {
+    return res.status(401);
+  }
+
+  const { oldPassword, newPassword } = req.body;
+
+  if (typeof oldPassword !== 'string' || typeof newPassword !== 'string') {
+    return res.status(422).json({ message: 'Unexpected data' });
+  }
+
+  try {
+    const usersList = await getUsersList();
+    const userIndex = usersList.findIndex((user) => user.username === session.user?.name);
+    const user = usersList[userIndex];
+
+    if (userIndex === -1 || !user) {
+      return res.status(400).json({
+        message: 'User is incorrect. Please, logout to update your session.',
+      });
+    }
+
+    const isValidPassword = await verifyPassword(oldPassword, user.password);
+    if (!isValidPassword) {
+      return res.status(400).json({ message: 'Old password is incorrect.' });
+    }
+
+    const newPasswordHash = await hashPassword(newPassword);
+    const updatedUsersList = usersList.map((user, index) =>
+      index === userIndex ? { ...user, password: newPasswordHash } : user
+    );
+
+    await updateUsersList(updatedUsersList);
+
+    return res.status(200).json({ message: 'Successful API send' });
+  } catch (error: any) {
+    console.error(error);
+    return res.status(500).json({
+      status: 500,
+      message:
+        error.code === 'ENOENT'
+          ? 'No such file or directory'
+          : 'API error, contact the administrator',
+    });
+  }
+}
diff --git a/pages/api/account/updateUsername.js b/pages/api/account/updateUsername.js
deleted file mode 100644
index 18077c6..0000000
--- a/pages/api/account/updateUsername.js
+++ /dev/null
@@ -1,84 +0,0 @@
-//Lib
-import { promises as fs } from 'fs';
-import path from 'path';
-import { authOptions } from '../auth/[...nextauth]';
-import { getServerSession } from 'next-auth/next';
-
-export default async function handler(req, res) {
-  if (req.method == 'PUT') {
-    //Verify that the user is logged in.
-    const session = await getServerSession(req, res, authOptions);
-    if (!session) {
-      res.status(401).json({ message: 'You must be logged in.' });
-      return;
-    }
-
-    //The data we expect to receive
-    let { username } = req.body;
-
-    //Read the users file
-    //Find the absolute path of the json directory
-    const jsonDirectory = path.join(process.cwd(), '/config');
-    let usersList = await fs.readFile(jsonDirectory + '/users.json', 'utf8');
-    //Parse the usersList
-    usersList = JSON.parse(usersList);
-
-    //1 : We check that we receive data.
-    if (!username) {
-      //If a variable is empty.
-      res.status(400).json({ message: 'A field is missing.' });
-      return;
-    }
-
-    //2 : control the data
-    const usernameRegex = new RegExp(/^[a-z]{5,15}$/);
-    if (!usernameRegex.test(username)) {
-      res.status(400).json({
-        message: 'Only a-z characters are allowed (5 to 15 char.)',
-      });
-      return;
-    }
-
-    //3 : Verify that the user of the session exists
-    const userIndex = usersList.map((user) => user.username).indexOf(session.user.name);
-    if (userIndex === -1) {
-      res.status(400).json({
-        message: 'User is incorrect. Please, logout to update your session.',
-      });
-      return;
-    }
-
-    //4 : Change the username
-    try {
-      //Modify the username for the user
-      let newUsersList = usersList.map((user) =>
-        user.username == session.user.name ? { ...user, username: username } : user
-      );
-      //Stringify the new users list
-      newUsersList = JSON.stringify(newUsersList);
-      //Write the new JSON
-      await fs.writeFile(jsonDirectory + '/users.json', newUsersList, (err) => {
-        if (err) console.log(err);
-      });
-      res.status(200).json({ message: 'Successful API send' });
-    } catch (error) {
-      //Log for backend
-      console.log(error);
-      //Log for frontend
-      if (error.code == 'ENOENT') {
-        res.status(500).json({
-          status: 500,
-          message: 'No such file or directory',
-        });
-      } else {
-        res.status(500).json({
-          status: 500,
-          message: 'API error, contact the administrator',
-        });
-      }
-      return;
-    }
-  } else {
-    res.status(405).json({ message: 'Bad request on API' });
-  }
-}
diff --git a/pages/api/account/updateUsername.ts b/pages/api/account/updateUsername.ts
new file mode 100644
index 0000000..c4e04b9
--- /dev/null
+++ b/pages/api/account/updateUsername.ts
@@ -0,0 +1,65 @@
+import { getUsersList, updateUsersList } from '~/helpers/functions';
+import { authOptions } from '../auth/[...nextauth]';
+import { getServerSession } from 'next-auth/next';
+import { UsernameSettingDTO } from '~/types/api/settings.types';
+import { NextApiRequest, NextApiResponse } from 'next';
+import { ErrorResponse } from '~/types/api/error.types';
+
+export default async function handler(
+  req: NextApiRequest & { body: UsernameSettingDTO },
+  res: NextApiResponse<ErrorResponse>
+) {
+  if (req.method !== 'PUT') {
+    return res.status(405);
+  }
+
+  const session = await getServerSession(req, res, authOptions);
+  if (!session) {
+    return res.status(401);
+  }
+
+  //The data we expect to receive
+  let { username } = req.body;
+
+  if (typeof username !== 'string') {
+    return res.status(422).json({ message: 'Unexpected data' });
+  }
+  const usernameRegex = new RegExp(/^[a-z]{5,15}$/);
+  if (!usernameRegex.test(username)) {
+    res.status(422).json({
+      message: 'Only a-z characters are allowed (5 to 15 char.)',
+    });
+    return;
+  }
+
+  try {
+    const usersList = await getUsersList();
+    const userIndex = usersList.findIndex((user) => user.username === session.user?.name);
+
+    if (userIndex === -1) {
+      return res.status(400).json({
+        message: 'User is incorrect. Please, logout to update your session.',
+      });
+    }
+
+    if (usersList.some((user) => user.username === username)) {
+      return res.status(400).json({ message: 'Username already exists' });
+    }
+
+    const updatedUsersList = usersList.map((user, index) =>
+      index === userIndex ? { ...user, username } : user
+    );
+    await updateUsersList(updatedUsersList);
+
+    return res.status(200).json({ message: 'Successful API send' });
+  } catch (error: any) {
+    console.error(error);
+    return res.status(500).json({
+      status: 500,
+      message:
+        error.code === 'ENOENT'
+          ? 'No such file or directory'
+          : 'API error, contact the administrator',
+    });
+  }
+}
diff --git a/types/api/settings.types.ts b/types/api/settings.types.ts
new file mode 100644
index 0000000..febfda8
--- /dev/null
+++ b/types/api/settings.types.ts
@@ -0,0 +1,12 @@
+export type EmailSettingDTO = {
+  email: string;
+};
+
+export type UserSettingDTO = {
+  username: string;
+};
+
+export type PasswordSettingDTO = {
+  oldPassword: string;
+  newPassword: string;
+};