Hardware resetting Nokia Lumia 920 is pretty simple just like its predecessor Nokia N8. All you need to do is that press the Volume Down and Power buttons together for 5 secs or until the phone's screen goes black. After that simply power on the phone as you normally do. This should not remove any of your data from the phone or even reset your Live account.
Hard reset is required when your phone hangs or otherwise and since Nokia Lumia 920 comes with a non-user serviceable battery, you cannot disconnect it. Hence, the phone has another circuit that is built to hardware reset the phone.