how to upgrade MacBook Air

The model year of a MacBook Air is available at Apple menu > About This Mac.

A late-2010 through mid-2011 MacBook Air can be upgraded to High Sierra 10.13.6 but no later version.

A mid-2012 MacBook Air can be upgraded to Catalina 10.15.7. See: https://support.apple.com/en-us/HT211683

A mid-2013 or later can be upgraded to the current version of Big Sur. See: https://support.apple.com/en-us/HT201475