The highest possible version of Safari with Catalina 10.15.7 is Safari 15.6.1
To update Safari beyond that you would need to upgrade your macOS if you can.
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 download and install macOS - Apple Support
A mid 2014 and late 2014 iMac can be upgraded to the latest version of Big Sur macOS 11 but no later. See: How to download and install macOS - Apple Support
A late 2015 iMac can be upgraded to the latest version of Monterey macOS 12 but no later. See: How to download and install macOS - Apple Support
A 2017 iMac can be upgraded to the latest version of Ventura macOS 13 but no later. See: How to download and install macOS - Apple Support
A 2019 or later iMac can be upgraded to the current version of Sonoma macOS 14. See:How to upgrade to macOS Sonoma - Apple Support
If you cannot or don't want to upgrade beyond Catalina, another option is to use a 3rd party browser that supports running their latest version, instead of using Safari. One such option is Chrome: https://www.google.com/chrome/