My Windows 10 Version 20H2 will not install the update 0x80004005.

Well this one REALLY annoyed me!  I've been an MCP for a couple of decades, back even as far as Windows NT/98.... yep.  Old and cagey and this stumped me until I read hints about the boot configuration data being hosed.

First, let me say this was a brand new PC, new build, new drives, and NEW Media Creation tool USB bootable drive.  It seemed ok at first until I got notifications about EOL of my version of Win10 that didn't make sense, and I couldn't get the Windows Update or Windows Update Assistant to do anything, it said I was entirely "up to date" on version 1909 that my machine had finally "upgraded" itself to via Windows Update.

My fix:   I booted into command prompt/admin as many of you should know how to do. 

Boot to Advanced Startup Options in Windows 10 | Tutorials (tenforums.com)

And I successfully backed up, exported, my old BCD.  

But instead of commands to rebuild or create a BCD anew, I did the super-secret-special-sauce version of 'sfc'.  Really, there is a special mode -- when I ran sfc /scannow it didn't fix things, but when I added two parameters, it worked like a charm:  How to Fix winload.efi missing or corrupt error in Windows 10 | Tutorials (tenforums.com) <-- I just did the part that's buried, "Run SFC and CHKDSK", just sfc.

The tricks are: 

a) find out what the command line thinks your Windows boot drive is.  If you're having problems, it may well be like mine, things were tidy.  Running bcdedit command will show you the drive

b) using "sfc /scannow /OFFBOOTDIR={yourdrive}:\ /OFFWINDIR={yourwindowsdriveandpath}

After rebooting, I ran the manual Windows Update Assistant tool and this time it worked as designed:

Download Windows 10 (microsoft.com)

I'd never had much luck with sfc before but I sure did this time.

YMMV -- GOOD LUCK!

Sorry it took me so long but here is the link to the page with the installer on it. Its the Media Creation tool. It WORKS and is LEGIT> It just takes your PC, out of commission for a few hours while it installs it.

https://appuals.com/update-20h2-wont-install-on-windows-10/

Its shown about half way down.

It's at least reassuring to know I'm not the only one having this issue, I try and try and try again to update and yet I can't get it to work

for me it gets to 94 percent installed and then it freezes without fail I've tried just about everything to get it to finish but with no ground gained

I was having the same problem with a Toshiba Satellite Pro laptop originally running Windows 7 professional that I was given as a gift by my employer, my attempts to do a clean install of the free version of Windows 10 home resulted in all sorts of continuing problems with updates.  I then decided to reinstall Windows 7 from the original back up disks provided by Toshiba, in the mean time using a different computer I downloaded Windows 10 direct from Microsoft onto a 32Gb SD card (you need at least 8GB of clean install space on a newly formatted card) inserted this into the SD card slot on the problem computer after Windows 7 had fully installed then open the SD card from the home screen and select setup it should then install the new Windows 10 over the top of the original Windows 7 retaining all previous drivers etc.  It appears to be the case that if the computer was running a professional version originally then then same version of Windows 10 needs to be installed to solve the problem.

HERE https://answers.microsoft.com/en-us/windows/forum/all/new-hp-envy-lapto-keep-asking-me-to-restart-to/99d6c855-9cf9-404c-b582-faf65b251ba0

ohh..thats great, gud for you