iMac late 2013 Version 21.5

Depends on whether your machine was starved for RAM in the first place. That iMac has two sealed-in RAM slots and can take 8 GB or 16 GB of RAM. Since the slots are sealed in, and the chassis is not easy to open and reseal, upgrading the RAM is a job for a repair shop or skilled technician.

If you run Activity Monitor and watch the Memory Pressure graph while you are running a normal workload, you can use the color-coding on the graph as a guide:

• Green means your system has adequate RAM for what it is doing
• Yellow means that your system has marginal RAM for what it is doing
• Red means that your system has inadequate RAM for what it is doing, and this may hurt performance

If you see lots of Green on the Memory Pressure graph, installing more RAM might not do much of anything, except to make your wallet that much lighter.

A Late 2013 21.5" iMac supports a maximum of 16GBs. It also requires it be taken apart to reach the memory slots.

Just to add, it can't run a version of macOS newer than Catalina 10.15.7. Time to think about a newer Mac.