you have to turn off the phone and remove the battery,
put the battery in again, hold volume up and plug in the USB cable, you'll see S/W Upgrade. (If you see the LG logo repeat again) .
Cheers. Good luck.
Edit: for the second method you must enter the code in your phone as if you were doing a call.
Sent from my LG P76020p-EUR-XX unlocked and rooted