I've downloaded the DP 8.1 Update 1 update on my Lumia 920. It also completed the preparing to install process, but on choosing to install, the gears appear, and after about 30 seconds without any progress with the installation being made, the phone reboots and displays an error:
Can't update phone
It's a probably a temporary problem. To try again, go to Settings > Phone Update. (0x80188308).
I've tried three times with the same result on each occasion. The phone is a Nokia Lumia 920, running 8.1 DP and Black. It was originally on EE, but I've unlocked it so I can use it on Tesco. I've got 4.7GB free. It isn't yet clear what about the phone might cause the error, and if there is a fix available. I haven't yet tried resetting (hard or soft).