Deleting a user on iMac
Can't delete myself as a standard 'user' on a 2015 iMac. Signed in with Administrator's credentials (iMac owner) and password. Opened preferences, opened users and groups, unlocked screen, selected my id, selected minus sign to delete, typed in correct id. Get a message password is wrong. Will not let me go beyond that point.
Trying to strip down the computer so owner can give it to his niece. Owner has purchased a new iMac.
Concerned if I strip down the machine and reinstall the operating system without deleting myself as a user, both his iCloud account and mine may be corrupted or deleted??
How should I move forward with this task?
iMac 21.5″, 12.2