Update mid-2012 MacBook Air

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 (13-inch, Mid 2012)

Model Identifier: MacBookAir5,2

Part Numbers: MD231xx/A, MD232xx/A

Newest compatible operating system: macOS Catalina 10.15.7

Follow Update macOS on Mac - Apple Support.

If your Mac can run a macOS version earlier than Apple's latest release, that Support document explains how to upgrade to an earlier version of macOS: How to get old versions of macOS - Apple Support.