Did you just shutdown to update? If so, I know Windows Update will download the updates in that setting, so the speed may be dependent upon your internet service speed. I doubt that you would break anything if you just power it off, though. You may have to do a recommended system check upon reboot. In the future, I suggest downloading and trying to install updates from within the user's desktop environment. If you are in your desktop environment during the update process, you can kill it from a command window with administrative access with the command:
net stop wuauserv
and restart it with:
net start wuauserv
So if the super long update is really bothering you, I think you will be okay stopping Windows Update or manually shutting down