I've never done it on GLiNet I've just flashed my OpenWRT One to update.

Find yours on https://openwrt.org/. You'll have to do some digging on that site but the docs are pretty decent IMO and search works well enough.

Generally you can flash back to the original OS, I can't think of an instance where you can't for any firmware flashing. The GLiNet default firmware is a proprietary fork of OpenWRT so it's already pretty similar. I brought them up since they're generally pretty powerful for the price and take to flashing well from what I've heard in the OpenWRT forums.