I want to upgrade my MacBook Air OS

Which MacBook Air model do you have, and which macOS is installed?

The built in hardware has to support further upgrades.

Did you already have a look at this related article on how to upgrade?

How to download and install macOS - Apple Support

Here's an Apple Support article that will shed some light regarding which MacBooks support which MacOS versions: Find out which macOS your Mac is using - Apple Support