If may iMac 27-in Late 2012 is not compatible to upgrade to a higher OS how do I upgrade?

You can upgrade to macOS Catalina, and no further with the late-2012 iMac. The following How to get old versions of macOS - Apple Support site provides the hardware requirements and Mac App Store download link for Catalina. Recommend that you use only the Safari browser for this.

Catalina does not support any 32-bit applications, or Microsoft software older than the current Microsoft 365 subscription, Office 2019 for Mac, or Office 2021 for Mac. You may want to run the free Go64 application to identify any 32-bit applications on your current operating system, and also verify that the printer/scanner vendor does provide current drivers for Catalina.

Be certain to make a last Time Machine backup of your current operating system before any upgrade, and then do not reuse that same Time Machine drive for the new operating system backup.

If you have a need to go beyond 10.15.7 then it will be time for you to replace your 2012. Please remember that by today's standards it its quite old and has old components in it that can wear out such as a spinning HD or Fusion Drive. If you want to keep it going awhile longer simply keep it in good shape, when the HD does die (they all do) replace it with an external SSD.

Above all, please backup daily using Time Machine!!!

A late 2012 iMac can be upgraded to Catalina 10.15.7 but no later -- you cannot upgrade it beyond Catalina. See: How to get old versions of macOS - Apple Support