Is there anything I can do to upgrade my os to the most updated one if my mac isn

There is nothing you can do about that. You can only upgrade your model to the last version of macOS with which it is compatible. Your profile says you are running 10.11. If your iMac is Late-2009, then it can be upgraded to 10.13.6 - High Sierra. There are no 21.5" iMacs prior to that model. How to upgrade to macOS High Sierra.

I'm assuming you cannot upgrade to either Mojave or Catalina which would require a Late-2011 or later iMac.

You'll never find out how to do that here.

The internet has all the answers.