Ive been suffering from this failed to update for two weeks now.
It takes like 10-20mins for the update to be done but after 100% it says reverting back because it failed to update.
I have tried to update for 10times already but it always fail.
while windows is running on PC with problems, put USB in and open file exploder (win key + E)
navigate to usb and run setup
this will offer to update current PC and should install version 21H1 on and in process fix the update problem by replacing windows.
you can use media creation tool to directly update PC but if it has update problems, safer to use the USB as you only have to download it once.