My friend gave me this iMac 21.5 but he and his son can't remember the administrator username nor the password that goes to it. I can only use the guest user login for now, but want to be able to sign in under my apple account.
Earlier Mac models
To properly prepare an iMac, the original owner needs to use the following directions.
What to do before you sell, give away, trade in, or recycle your Mac - Apple Support
Then you can use your Apple ID to setup the iMac and make it yours.
To properly prepare an iMac, the original owner needs to use the following directions.
What to do before you sell, give away, trade in, or recycle your Mac - Apple Support
Then you can use your Apple ID to setup the iMac and make it yours.
If your friend has forgotten the local Admin account password, but knows his Apple ID and Apple ID password , he may be able to reset the local Admin account password using the instructions here.
Apple Support – If you forgot your Mac login password
Once he has access to the local Admin account again, he can follow the directions for what to do before giving the Mac away.
Erase and re-install it:
… What to do before you sell, give away, trade in, or recycle your Mac - Apple Support
if you don’t do that, you won’t be able to update it, or update its apps, and you’ll be chasing old settings and old customizations and old installed apps ~forever.
Can I change administrator on a gifted iMac without password?
