how to upgrade

Your MacBook Air can upgrade from Sierra to High Sierra because of the same hardware requirements. If your MacBook Air is a 2012 or newer model, it can upgrade to:

• Mojave
• Catalina (64-bit applications only)

Only use the Safari browser when accessing the preceding links, or you will have issues acquiring the upgrades. Never perform an upgrade without a Time Machine backup of the previous operating system beforehand.

If you want to upgrade to Catalina, that takes some planning about your current third-party applications and certain Apple applications (e.g. iWork '08/'09 and iMovie '09), and whether they are 32 or 64-bit applications, and whether the printer/scanner vendors offers 64-bit drivers and support software for Catalina. I suggest that you use St. Claire Software's Go64 application to determine the 32/64 bit application question.