how do I add more memory to my iMac?

Welcome to the Apple Support Communities.

Adding more RAM may or may not help. To see what will, I believe we need to proceed with an EtreCheck report. EtreCheck is a useful App to diagnose and identify issues, both hardware and software. It is trusted by many users. Please follow the steps outlined below:

1. Navigate to: https://etrecheck.com and get the free version of EtreCheckPro.
2. Download it. It is a free and trusted app. It is often recommended here on Apple Support Communities. 
3. Open EtreCheck, and before running the report, be sure to check “Allow full Drive Access” at the bottom of the EtreCheckPro window.
4. Run the report, and put it in a post, as seen here: https://discussions.apple.com/docs/DOC-250000211. We will then examine the report for issues in your Mac.

Jack

➡️ Time Machine has not run in a long time. Please let Time Machine backup.

➡️ Your Security and Updates are disabled. Please go to System Preferences > Software Update, and turn on the following:

• • Check for Updates
  • Download new updates when available
  • Install App Updates from the App Store
  • Install System data files and security updates

Then, go to System Preferences > Security & Privacy > General, and make sure the "Allow Apps downloaded from" is set to either "App Store" or "App Store and Identified Developers" (that may be the only options shown).

➡️ Delete Symantec/Norton ASAP! [Instructions]

➡️ Delete Norton VPN.

Next, please use DriveDx to check the health of your Fusion Drive setup and upload those reports.

Jack

Your Fusion Drive is failing. Make sure you have a backup ASAP!

You essentially have two options from here:

Option 1 - Use an external SSD as your startup disk to boot your Mac to "replace" your failing Fusion Drive. You'll also see a speed improvement. For more info, instructions, and what external SSD to buy, please read: How to Setup and Use an External SSD as y… - Apple Community.

Option 2 - Since your Mac is an older Late 2012, you could consider replacing the computer with a new Mac. A new 24" M1 iMac would be a great replacement.

Cheers,

Jack

27" memory upgrades fairly easy and straight forward. See: https://support.apple.com/en-us/HT201191

Memory is best bought from Other World Computing…

https://eshop.macsales.com/shop/memory/iMac/2012_27/DDR3L

Could you please upload the text version of both Macintosh HD reports as shown in the User Tip?