how do I update my Mac to Monterey? run on Catalina 10.15.7

Need to know what specific model iMac you have. Please go to the Apple menu and About this Mac. It should say iMac and list the size and year. Your signature says iMac Line (2012 and Later) but is not specific.

Monterey requires a Late 2015 or newer iMac.

Catalina 10.15.7 is the latest version of macOS you can run.