Before you start using your new Office 365 account on you iPhone or iPad, you'll have to remove your old account, or it will automatically get logged in when you download Outlook.

To do this just follow the steps below:

  1.  Open the settings app
  2. Tap "Passwords & accounts" above mail. Some iPads/iPhones may have this named Mail/Mail, Contacts, Calendars or Accounts.
  3. You should have the list of accounts added to your device. Tap the email you wish to remove, and you will be shown more details.
  4. Then press the Delete Account button at the bottom of the page.
  5. Confirm this by pressing "Delete Account", or for some versions "Delete from my iPhone".
  6. Once you have done that, you can add your new Office account.