how to i upgrade from sierra to catalina

If it was mine I would upgrade it to Monterey (the latest version it supports). See: How to download macOS - Apple Support

Also note that if you upgrade to Catalina or later, if you have any non-apple 32-bit apps, yo will also need to update them. You can use the following to identify them: https://www.stclairsoft.com/Go64/

A 2017 MacBook Air can be updated up to macOS 12 Monterey.

You can download Catalina and Monterey from the Mac App Store by using the links here: link-> How to download macOS - Apple Support