how to update macbook air 2012

A mid 2012 MacBook Air can be upgraded to Catalina 10.15.7 but no later. See the following from Identify your MacBook Air model - Apple Support

MacBook Air (11-inch, Mid 2012)

Model Identifier: MacBookAir5,1

Part Numbers: MD223xx/A, MD224xx/A

Newest compatible operating system: macOS Catalina 10.15.7

If not already upgraded to that, see: How to download macOS - Apple Support

It can't be upgraded past Catalina.

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