Procedure to Soft or Hard Reset (Factory Reset) your Nokia Lumia 920 Windows Phone.
Soft Reset Lumia 920
If your battery is not removable, switch the power on and disconnect the charger.
Then press and hold the power key + volume down key for about 15 seconds until the phone vibrates and restarts.
Hard Reset Nokia Lumia 920:
Then press and hold the power key + volume down key + Camera Key for about 15 seconds until the phone vibrates and restarts.
Then release the Power button and keep the volume down + camera button down for another 5 seconds.
Now Your phone will be reset to factory Default state.
*This will erase all your data