Microsoft's first cumulative update for Windows 10 - KB3081424 - is causing havoc for some users.
The problem, in a nutshell, is that the update puts affected systems into an endless crash loop. The update tries to install, gets to a certain point, fails, and then displays the unhelpful "We couldn't complete the updates, undoing the changes."
I found a fix, but it involved some registry editing voodoo to remove legacy junk (which in my case was related to an Nvidia driver installed on the machine prior to upgrading to Windows 10), a task that some of you might not be comfortable with. Also, since this is not an officially sanctioned fix I'm not comfortable outlining the process here. That said, the fix that worked for me is described in a thread on the Microsoft Answers community if you want to give it a go.
Windows 10 cumulative update causes 'reboot loop' havoc for some users | ZDNet