I have an iMac 21.5-inch, Late 2015 desktop computer and want to get more Ram for it. I’m confused as to what I should buy and if I can install it myself. It currently has 8 gig of Ram but I want to get 16 or 32 gig. Does anyone have suggestions?

RAM cannot be upgraded on your Mac.

Are you having performance issues? If so, 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.

Cheers,

Jack

Are you having performance issues? Is that why you wanted more RAM?

Memory on that model iMac is not considered user-replaceable. Its maximum memory configuration is 16 GB.