It's not that easy, once updated you cannot simply downgrade. Maybe you've recieved a new version of android too. This is typically when the software changes looks/format. Without knowing the details you will not be able to downgrade back to the previous version of android. The long version is, when a new version of software is applied it sometimes upgrades the bootloader on the phone. This typically cannot be undone and can/will prevent you from downgrading. Unless you can unlock the bootloader. Which will wipe all information off your phone. There are ways to prevent updates, but I wouldn't recommend doing so.
