MacBook Air will not update past Sequoia 15.8

A 2020 MacBook Air can be either an Intel model or an M1 model, and that determines how far you can upgrade macOS.

Apple lists the MacBook Air (M1, 2020) as compatible with macOS 27 Golden Gate, which is currently the latest major version of macOS. The Intel-based MacBook Air (Retina, 13-inch, 2020) is not compatible with macOS Tahoe 26 or macOS 27 and has macOS Sequoia 15 as its newest compatible major version. (Apple Support)

To check which model you have, choose  > About This Mac.

If it says Processor: Intel, macOS Sequoia 15 is the newest major macOS version available for your Mac.

If it says Chip: Apple M1, your Mac is compatible with macOS 27 Golden Gate. (Apple Support)

Apple currently lists macOS 27.0 Golden Gate as the latest major macOS, macOS Tahoe 26.7 as the latest version of Tahoe, and macOS Sequoia 15.8 as the latest version of Sequoia. (Apple Support)

If yours is the M1 model, go to  > System Settings > General > Software Update. Software Update will offer only versions that are compatible with your Mac. (Apple Support)

Apple's instructions for upgrading are here: How to upgrade to macOS 27 Golden Gate

Apple's model-identification page is here: Identify your MacBook Air model

Your 2020 MacBook Air with the 1.1 GHz dual-core Intel processor cannot officially run macOS Tahoe 26. 

Apple announced in June 2020 that it was transitioning the Mac from Intel processors to its own Apple silicon, while stating that Intel Macs would continue receiving macOS updates for years. 

Your Mac is now at the end of its major macOS upgrade support, with macOS Sequoia 15 being its final officially supported major release.

You should therefore install the latest Sequoia update offered by Apple. Go to  >> System Settings >> General >> Software Update. 

About the security content of macOS Sequoia 15.8.1

If an update is not offered, consult Apple’s macOS update troubleshooting guide and macOS security updates.

Apple’s original announcement of the Intel-to-Apple-silicon transition is available here: Apple announces Mac transition to Apple silicon.

That is correct. For an Intel-based MacBook Air (Retina, 13-inch, 2020), macOS 15.* (Sequoia) is the end of the line. As to what to do, as long as the computer meets your needs, just keep using it.

Safari is still one of the "most recent three" that

• Receives current versions of Safari
• Is supported by vendors such as Microsoft and Adobe

and will be for another year. Even after that, it may be able to run current versions of programs such as the Firefox browser and the LibreOffice office suite for years to come.