To which macOS can I update my 2014 MacBook Air?

Yes, a 13" 2014 MacBook Air is compatible with macOS Big Sur.

Ref: macOS Big Sur is compatible with these computers - Apple Support

How to download and install macOS - Apple Support

I don't remember going straight from High Sierra to Big Sur being a problem when I did that upgrade on my 12" Retina MacBook. (Other than the expected loss of support for 32-bit applications.)

My MacBook had 8 GB of RAM. I don't know if your Early 2015 MacBook Air has 4 GB or 8 GB. If it only has 4, that might affect how long it takes to do the upgrade (though I'd expect to have a bigger effect on how quickly you'd be able to run applications).

All of the Early 2014 MacBook Airs had internal SSDs – there were no mechanical drive or Fusion Drive options. So, unless your SSD is nearly full, I don't think the speed of your internal drive would be causing the upgrade to take a long time.

Now that you're on Catalina, you could try updating to Big Sur again.

If you're afraid that the update will get stuck, you could clone your Catalina installation to an external drive, install Big Sur as an upgrade on that external drive, and then clone the Big Sur installation back if it succeeds.

MetroIsW wrote:

And if something goes wrong can i reinstall mac?

What I would suggest is to use Carbon Copy Cloner or SuperDuper! to make a bootable clone backup of your current system on a spare external drive BEFORE you do any upgrade. That way, if something goes wrong, you'll have something that you can boot from, and use to reset the internal drive (e.g., clone things back from the external to the internal).

There is Internet Recovery, but that only gets you an OS back – not your data. (Assuming that it works, which for older versions of macOS, might not be a given these days.)

If you don't want your data, you can do a "clean install" of the operating system, where you erase the destination drive before running an installer against it. Not wanting your data is kind of an odd request, because most people prefer to keep theirs.

You're still going to need a bootable drive to work from.