how do i upgrade my mac from mac OS catalina 10.15.4 to the current program

The model year of an iMac is available at Apple menu > About This Mac.

A late 2012 through late 2013 iMac can be upgraded to Catalina 10.15.7 but no later. See: How to get old versions of macOS - Apple Support

A mid 2014 and late 2014 can be upgraded to the latest version of Big Sur macOS 11 but no later. See: How to get old versions of macOS - Apple Support

A late 2015 or later can be upgraded to the current version of Monterey macOS 12. See: How to upgrade to macOS Monterey - Apple Support

Safari is upgraded when you upgrade your macOS as described in my post above.

If you cannot upgrade macOS due to the age of the computer, another option is to install a 3rd party browser. The latest version of Chrome, for example, will run on OS X 10.11 or later. https://www.google.com/chrome/