Unable to update iMac due to previous owner's Apple Account
I was gifted an iMac a little over 2 years ago. It still has the owners Apple ID attached so I’m unable to update as that’s the primary account on the device. How do I fix this so I can use it?
[Re-Titled by Moderator]
Original Title: Unable to update
iMac 21.5″, macOS 10.12