How can I go to a previous version of Android?

Brian has an Android phone and after an over-the-air update, it's running really slow. Leo says that it's probably due to a bad upgrade. He suggests doing a complete factory reset. If that doesn't work, downgrading is an option. Brian may have to root the phone and get a stock version of 4.2.2, though. Leo advises going to XDA-Developers.com. CyanogenMod is a good one. He should read up on this though, as not all mods work the same.