You can access the System Restore function from Safe Mode, or from the Advanced Startup Options if that doesn't work. In order to safeguard against this type of thing, Windows will usually create a restore point just before applying an update, so you can roll it back if there are any problems. After the update installs, you can restart Windows to exit Safe Mode.
Accessible from the Startup Settings menu, this mode disables everything but the absolute essentials Windows needs to run, ensuring that any software conflicts that could be interfering with the update are removed. Use your computer's physical power or reset button, and remember that you may have to hold it down. Resetting your PC mid-installation is a cardinal sin and expressly warned against by manufacturers, but if your update is genuinely stuck then you haven't got a lot of options.
