1. On the iPhone or iPad home screen Tap the Settings App.

  2. Scroll down and tap "Accounts & Passwords" or "Mail, Contacts, Calendars" if you're on an older device.

  3. Under accounts you should find the old email account you wish to remove, tap on it in the list.

  4. At the bottom of the next screen there will be a Delete Account button, tap this to remove the account.

  5. Once removed you can follow the instructions required to add your new email account.

    See Add an Office 365 email account - iOS Mail App