If a user account has existing data it is not possible for it to be deleted. We also don't advising amending an existing user account to a new user. Instead we recommend that you disable a user account if it is no longer needed.
Once you have navigated your way to the user record, via the Tools & settings icon > File maintenance > users, you can simply tick the Account disabled box:
This will free up any licenses being used by this account and hide this user from most live sections of the system.