How can I upgrade from El Capitan to a newer OS on my Macbook Air from 2015?

Hello!

I need guidance on how I can install newer OS versions on my Macbook Air. Im still running El Capitan due to an issue I once had when trying to download High Sierra at the time. I just couldn't bother trying to figure out what the solution was at the time, so I stuck with El Capitan.

Now, I need newer versions to download a few games and programs. Is there a way or is it too late? I've tried installing older versions on the App store, but it only downloads a small file, not the actual 10+ GB OS file.

Thx in advance!