I had exactly the same problem, tried every possible solution that has been suggested, northing appeared to work. I have two desktop PCs and a laptop. The laptop installed the update without any problem as did one of the desktop PCs, leaving one PC that just would not install the update.
However......it appears that at least part of the update DID install as when I compare the 'Start' and 'Action Centre' screens on the three computers they are now all the new start and Action Centre design!
There is no trace of the update in the update history apart from an entry that says 'install failed', but it certainly seems as though some part of the update has installed.