my macbook air 13 is not being able to download the new software
how to download the new software it still has the 10.14 version
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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: How to get old versions of macOS - Apple Support
A mid-2013 or later can be upgraded to the current version of Big Sur. See: How to upgrade to macOS Big Sur - Apple Support