My iMac won't accept my Apple ID password
I bought a MAC home computer from around 2015. I really have never used it and a prompt screen keeps popping up asking me for my apple ID password which I type in the correct one and it is not accepting it. So now, I can't get into my administration for info to update my computer.. What can I do?
[Re-Titled by Moderator]
iMac 21.5″, OS X 10.11