Welcome to Android Central! For which phone? If it's a Samsung, you'd have to use Odin to flash the previous firmware: http://forums.androidcentral.com/am...w-flash-stock-rom-via-odin-new-interface.html. However, the phone will then immediately prompt you to update again, and you probably won't be able to refuse it after a certain amount of time. You'd have to disable the system updater, which might be in Developer Options, but on most phones requires root.