i can not find instructions for upgrading my memory for my iMac (21.5-inch, Late 2012)

16GB is the maximum. However if you are using a 1TB HDD I'd suggest you first try reinstall MacOS onto an external USB 3.0 SSD.

Oversimplified:

1) remove screen

2) remove logic board...

As previously stated this isn't considered a user serviceable upgrade. In fact, some authorized technicians will refuse the job, so call ahead and make sure the AASP of your choice will even accept the work. If you are curious you can find guides by OWC on macsales.com, rated 'advanced'.

The RAM (memory) in that year model 21.5" iMac is not user upgradable.

It must be completely disabled and upgraded by an Authorized Apple Service Provider that is willing to do the work.

Welcome to the Apple Support Communities.

Sorry, you cannot upgrade the RAM yourself. Did you want to upgrade the RAM due to performance issues?

Jack