I search on internet but i did not find some easy way to install official firmware.
I'm sorry for my english but i hope you will find this thread very usefull.
Ok let's begin:
1. Charge the phone.
2. Open LG mobile support tool official.
3. Turn OFF the phone.
4. Press VOL- and plug the USB cable until you see S/W Upgrade.
5. LG tool will find your phone on 4.1.2. It OK.
6. Go to Options - Country and language and select your country and language.
7. Go to Option - Upgrade recovery.
8. Then the LG tool will search your phone firmware and will automaticaly download latest firmware and install .
9. The phone will reboot, and the logo LG will remain on screen, in my case. Do not panic. Unplug the usb cable and press power button to close the phone.Keep pressing untill is off.
10. Press VOL- and Power to enter recovery mod, do a factory reset, wait for the green robot to finish, restart and then the phone will boot official 4.1.2.
Now you have official 4.1.2 fast and easy.
Good luck !