I have a MacBook Air 11-inch, Mid-2012 I need to upgrade it to be able to upgrade Numbers, Garage Band and Keynote what do I need to do to be able to that?

MacBook Air upgrade:

A mid-2012 MacBook Air can be upgraded to Catalina 10.15.7 but no later version. See instructions in: How to get old versions of macOS - Apple Support

You can upgrade your mid-2012 Air as far as Catalina (10.15.7). That will get you the current Pages, Numbers, and Keynote (11.1), but not the current Garageband which requires Big Sur (11.0 or later).

Backup up High Sierra with Time Machine, so you can retreat from Catalina if you need too. As Catalina does not support any 32-bit applications, printer, or scanner drivers, you need to check which applications you currently have that are 32-bit. Use the free Go64 application to get a list of those. Check with your printer/scanner vendors to ensure that you have driver coverage on Catalina, or that your networked printer is supported by Apple's built-in AirPrint drivers instead of those from the vendor.

Using only the Safari browser, visit How to get old versions of macOS, and click the Mojave (32/64-bit), or Catalina link to their full installers in the Mac App Store. The downloaded upgrade installer will land in your /Applications folder.

You can upgrade to macOS Catalina on your mac as described above.

You will be able to install Pages, Numbers and Keynote as the minimum requirement is macOS Catalina,

but GarageBand requires a mac running macOS Big Sur 11.0, which your mac cannot run.

There is ta possibility that if you had downloaded GarageBand previously you can download

a Catalina friendly version of GarageBand through the App Store.

After installing macOS Catalina open the App Store click on Store in the menubar select View My Account,

sign in, this should open the Purchased apps page, if you ever had GarageBand in an earlier OS then it should

appear here available for download.