Hi, Hamsterray, your MacBook Air from 2015 should be able to upgrade/update to Catalina, according to this link from Apple : macOS Catalina is compatible with these computers - Apple Support (CA)
please note that Catalina breaks all support for 32- bit apps/ software/programs. If an app/piece of software/ program has all or some 32-bit stuff in it, it won’t work/run. You’d need to find a Catalina/64-bit compliant app/piece of software/program , or an equivalent/alternative for it to work in Catalina… you have been advised, that being said, Catalina should be in the App Store… use this link+ Safari to download it, as it goes directly to the App Store, other browsers may work, but you’ll have to point ‘em to the App Store for it to work, so here it is: How to download and install macOS - Apple Support
Oh, and Yosemite has a bug in it- wifi doesn’t work, so plug in your Ethernet/internet cable directly into your MacBook Air 2015 which you’ll probably need an adapter for+ set your Network to Ethernet, if you haven’t done so already
John B
john B