Soft- / Hardreset Nokia Lumia 920

Softreset

Press and hold the power button and volume down button for about 10 seconds will do the reset; the phone should vibrate and reboot your phone.

Hardreset

To hard reset hold the volume down, power button and camera button until the phone vibrates, then release the power button and keep the volume down and power button down for another 5 seconds.  Your phone should then reset to factory state.