UPDATE IMAC OS FROM MOJAVE

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

A late 2009 through mid 2011 iMac can be upgraded to High Sierra 10.13.6 but no later version. See: How to download macOS - Apple Support

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

A late 2015 iMac can be upgraded to the latest version of Monterey macOS 12. See:How to download macOS - Apple Support

A 2017 or later iMac can be upgraded to the current version of Ventura macOS 13. See: How to upgrade to macOS Ventura - Apple Support

Be aware that if you upgrade to Catalina 10.15 or later you will also need to update any non-Apple 32-bit apps. See: https://www.macworld.com/article/232735/how-to-check-if-mac-software-is-32-or-64-bit.html

You haven't said what model year of iMac you have. The model year of an iMac is available at Apple menu > About This Mac.

My previous post listed the latest versions various model years can be upgraded to, but you don't need to upgrade to the latest. You can click on: How to download macOS - Apple Support to upgrade to earlier versions.

See my previous warning about upgrading to Catalina or later. If you are upgrading to Catalina or later there usually isn't any reason not to upgrade to Ventura and there are security reasons in favor of Ventura.

You can upgrade it to any of the earlier versions using the instructions in How to download macOS - Apple Support or to Ventura as described in How to upgrade to macOS Ventura - Apple Support

If you don't upgrade to Big Sur, Monterey, or Ventura, all of which include current security updates for Safari, then I would recommend that you use a 3rd party browser that supports running their latest version.