I have an iMac 2010 21.5 inch that I would like to update, how can I do this?
iMac 21.5″, macOS 10.12
A late-2009 through mid-2011 iMac can be upgraded to High Sierra 10.13.6 but no later version. See instructions in: How to get old versions of macOS - Apple Support
If you have a need to go beyond 10.13.6 (which is now several generations outdated) then it is time to buy a newer Mac. You have many options available to you that will fit your needs If that is something you are open to. Please describe your computing needs for us and we can then offer some suggestions to you.
Updating iMac
