The latest cumulative update for Windows 10, 1511 KB3156421, seems to have a bit of an issue on a large number of systems when attempting to install it. Even though it was just released, it seems to have the reputation of failing and then rolling back completely. The cause is unknown, but a Microsoft engineer is quite interested in finding a solution to this persistent issue.
Windows 10 Cumulative Update KB3156421 is failing for a lot of people
The Microsoft software engineer in question points out that there could be several causes, and that at times it might simply be a temporary issue that resolves when you try again later.
“There are actually several different potential causes for this. Some of these are temporary (i.e. the update will install successfully on a later attempt) and some are not.”
The failure won’t brick or break your system, it just fails to install then restarts to the desktop. You can then opt to try again later or simply hold off until a new version that fixes the issues causing the inability to install.
If the latest cumulative update does indeed cause issues and you’re unable to even boot into your normal desktop, then you’ll have to boot into safe mode and then uninstall the update completely before restarting. Be cautious when updating, but just know that it won’t completely render your PC useless if you do.