SodBuster1's description helped me.
My Surface Pro 4, a flickergate beauty, could not get past 61%. I ignored the update for weeks but then other updates would not install pending 20H2 so I had to do something.
I downloaded the Update Assistant (easy search: Windows 10 Update Assistant), started the update and let it crank and crank. It slowed down around 80%, I thought it was frozen... but then it showed 100% (~40 minutes). It restarted and then it was done. Update complete.
My other Surfaces updated quietly and without a hitch.
Dear Joshu001, Thanks a lot. This worked like a piece of cake. I was in Windows version 1903 for a very long time. Wasn't able to perform windows update through Windows' default "Check for Update" screen. It was either stuck at Installation 61 % or threw a couple of different error codes. I tried with Wi-Fi, wired but all in vain. I tried troubleshooting, checking device manager for latest drivers, SFC scan and all that stuff.
Luckily I stumbled upon this post yesterday and read your comment. I immediately downloaded Windows Update Assistant (damn.. I never knew such a tool existed). I wanted to make to a point to comment here so others who are in trouble like I was can benefit.
This tool initially threw an error saying other installation window was up and running. It referred to the default "Check for Update" window. I killed that and restarted laptop couple of times and started running the tool afresh. I had my laptop wired for faster internet connection. It took a good 5-6 hours (I kept it overnight). In the morning, it was all done and I was upgraded from 1903 to 20H2.
THANKS A MILLION :)