Safari upgrade in Sierra 10.12.6

In order to update Safari you would need to upgrade macOS on your iMac.

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

If you cannot or don't want to upgrade your iMac macOS to Big Sur or later, for security reasons you should consider using a 3rd party internet browser that supports running a current version of their browser on your version of macOS. For example, the current version of Firefox will run on macOS 10.12 or later and the current version of Chrome will run on 10.13 or later. See: https://www.google.com/chrome/ or https://www.mozilla.org/en-US/firefox/new/