Can I upgrade the memory of my Late 2015 iMac?

You can upgrade the memory. See information in Install memory in an iMac - Apple Support

The following is a well regarded source for memory upgrades: https://eshop.macsales.com/shop/memory/iMac/Retina-5K2015/DDR3L

Whether you should depends in part on whether you want to spend more money on a 9 year old iMac. Note that you won't be able to upgrade the macOS beyond Monterey even if you add more memory.

You can.

If you intend to keep the iMac for the foreseeable future, you should seriously consider upgrading the RAM.

8GB is the bare minimum for today's macOS requirements.

If you click the 'Memory' tab at the top of that screen, I believe you'll find upgrade guidance for that iMac.

Yes you can however,

1) Adding more RAM will not make the Late 2015 iMac go any faster.

2) If anything, more RAM will only allow you to run more App's at at the same time.

3) It is getting a little late in the game to spend money on a RAM upgrade for that old of an iMac.

If you are experiencing generally slowness..?

A good way to find out what is causing slowness, is to

Download and run the free version of EtreCheckPro, from > https://etrecheck.com/en/index.html

Then post the Report, as per > How to use the Add Text Feature When Post… - Apple Community