If your phone doesn't want to boot up again after the update, it gets stuck in a black screen, follow this tutorial where we root the Galaxy SII with Odin and put an official Samsung Android 4 ICS on it.
At any given time you can reset your Nexus 4 smartphone to the factory default settings, meaning you will bring your phone back to the same state as it was on a day when you took it out of the box.
You just bought Nexus 4 and you don't know anything about Android operating system, so I will guide you through literally first steps on this great phone.
Nexus 4 smartphone comes with a bunch of Android widgets you can add to one of the screens. Let me show you how to add a widget to a screen and how to customize it.