Update macOS Version
How do I update from macOS Sierra 10.12.6 to 10.13 or a newer version?
MacBook Air 13″, macOS 10.12
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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: 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