When you are logged into webmail on your University of Derby account you can sign out of this account and log into a different Office 365 account.
Please Note: If you try and select sign out when you are in the mail app it might not sign you out - so go to 'Office 365' then sign out:
If you are in webmail (https://outlook.com/owa/derby.ac.uk )
- click on 'Office 365' in the top left of the screen
- then click on your profile picture in the top right and select Sign out
You will then be returned to the login page and can select 'Use another account'