First, check to see if your MacBook Air can be upgraded. See the following:
The model year of a MacBook Air is available at Apple menu > About This Mac.
A mid 2009 MacBook Air can be upgraded to El Capitan 10.11.6 but no later version. See: https://support.apple.com/en-us/HT211683
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/HT211683A 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