I have windows10 Installed on my SSD DRIVE. I want to update it, but I can't. Every time, the same PROBLEM occurs: my system restarts and after taking a long time, and in the end says "windows 10 can't complete update undoing changes". I've tried using Microsoft's "Windows 10 Update Assistant" and also updating the windows manually with CMD, but the same issue happens every time.

I've READ many answers on this issue on reddit and other websites and they didn't work...

I would appreciate if anyone could help me get rid of this problem.No, I do not know the best answer.
Do you have another copy of Windows 10 on yu PC?
If so, have you tried  this:
Disable or remove the SSD.
Set BIOS to boot from HDD.
Tun PC on and boot Windows from HDD and then try the update.

The objective is to find if the SSD installation prevents updates.

Elsewhere a few SAY they can not update using SSD as boot drive.
From Microsft:
https://answers.microsoft.com/en-us/windows/forum/windows_10-win_upgrade/windows-10-upgrade-and-ssd-boot-drive/fc6af58b-ec9b-4a18-8d74-2c3e0b765a49
Really?
https://answers.microsoft.com/en-us/windows/forum/windows_10-performance-winpc/windows-10-creators-update-crashes-ssd-boot-disk/40f1a26e-ff5f-4809-80b1-e0f3496d675a
I can not verify the above.  cared to share your solution - it may help others.