Boot your phone into download mode:
Put your G2 into download mode by switching it off, hold the Volume up button and insert the usb cable into the phone; wait for the drivers to finish loading if doing it for the first time.
Start the LG Mobile Support Tool and click on Options -> Upgrade Recovery
It should now download the firmware (about 2gb) and flash it - this may take some time.
This is needed if you stuck in a bootloop after the flash is done:
Factory reset (this wipe your data and sdcard):
1. Turn the phone off.
2. Press and hold the Power and Volume down button.
3. When the LG logo pops up let go for a second and press the combo again.
4. You'll be prompted to press the Power button 3 times to wipe the device.