how do i update my MacBook air it has Mac Os high sierra version.10.13.6

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. See: https://support.apple.com/en-us/HT211683

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

A mid 2013 and early 2014 can be upgraded to the current version of Big Sur. See: https://support.apple.com/en-us/HT211683

An early 2015 or later can be upgraded to the current version of Monterey. See: https://support.apple.com/en-us/HT212735