Is there an update to High Sierra 10.13.6 version?

10.13.6 is the last High Sierra version.

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 and install 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 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. See:How to download and install 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

If you can't or don't want to upgrade your macOS, for security reasons you should consider using a 3rd party internet browser that supports their current version with your older macOS. One example is Chrome if you have macOS 10.13 or higher. See: https://www.google.com/chrome/

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.stclairsoft.com/Go64/

or

 https://www.macworld.com/article/232735/how-to-check-if-mac-software-is-32-or-64-bit.html

Apple doesn't charge for the upgrade.

You may have a cost if any of your current applications aren't compatible (see the last paragraph of my previous post) or if you have a printer or some other device that isn't compatible.

This is the latest I can find

Security Update 2020-006 (High Sierra)