what is the latest update for a 21.5 inch late 2013 IMac which currently runs Yosemite 10.10.5?

You can get Catalina using this link: How to get old versions of macOS - Apple Support. Choose Catalina from the list. Always backup before updating!

Cheers,

Jack

Some tips:

• If you have older critical-meed software installed, do Mojave10.14 instead as an intermediate step. 10.15 Catalina will not run any older apps like old Adobe CS or pre-2016 versions of Office Mac. You can then use this free App to see which of yours will NOT work with Catalina: https://www.stclairsoft.com/Go64/
• Use Safari for macOS system downloads. There are barrels of reports here of Firefox not being able to do so.
• If your see an advisory about a file being damaged, it is an over-the-top way of telling you that an older and out-of-date version of the installer is already in your Applications folder. Simple delete it and empty the Trash.

Catalina 10.15.7