iMac 21.5 2011 os10.13

On your version of macOS, the way to get the free update is through the App Store. Launch that and do a search for Catalina in the search field in upper left corner. It is a free upgrade.

Here is a support article with more information: https://www.apple.com/in/macos/how-to-upgrade/

Please be sure to make a full backup of your current hard drive. (Either with Time Machine, or a different service of your choosing)

See Step 2 in these instructions for how to backup your Mac: https://support.apple.com/en-us/HT201475

That model of iMac can only upgrade to what you are currently running: 10.13.6. You can't go any further.

Old Toad is 150% correct!

That makes up for all the times I was only 50% right. 😉

😃