The official firmware files for Android Marshmallow update for the LG G4 are now available as TWRP-flashable ROM, with due credit to recognised XDA developer, autoprime. The flashable files are from build version 20A that LG recently released. The flashable files include system, boot, modem and bootloader zips.
Autoprime has also posted the KDZ file for the Marshmallow update, but if you don't want to take the pain of flashing the KDZ file using LG flashtool, which is a slightly lengthy method, you can always make use of the TWRP-flashable files, as the process is quite simple and easy as you do with any other flashable zip.
He further warns: "Notifications may not work with all apps. To check for possible missed notifications go to Settings, battery/energy saving, battery consumption, ignore optimization. Knock code has been removed but will return in a future update."
If you still have not received an OTA notification for the update, consider trying out the method below to install stock Android Marshmallow update on G4 manually. All you need for this is an LG G4 with TWRP (recommended TWRP 2.8.7) installed and its bootloader should be unlocked. Click here for a guide to unlock the bootloader of your G4.
The stock ROM files are compatible only with the G4 model H815, which is the international variant. Before you proceed with the guide, ensure that you have created a backup of all data of your phone.
How to install stock Android 6.0 Marshmallow on LG G4
Step 1: Connect your phone to PC and transfer the downloaded files to the phone's storage and remember the location
Step 2: Disconnect the phone and put it into recovery mode. You might use Quick Boot app from the Play Store for this. Just download and install the app on your phone. Then launch it and follow the on-screen prompts to boot into recovery. Alternatively, use the following hardware key combinations:
- First switch off the phone
- Press and hold Volume Down and Power buttons together for a few seconds until you see the LG logo on the phone's screen
- Release the buttons for a second and then hold them back again
- The recovery mode menu should now appear
- Use the Volume Up/Down key to navigate between the option and Power button to highlighting
- You phone will now enter recovery mode
Step 3: In recovery mode, create a backup of all data of the phone. From recovery tap on Backup, then do a swipe on the bottom of the screen to confirm the process
Step 4: Perform a Factory reset. Select Wipe, Advanced wipe, then select Dalvik Cache, System, Data and Cache, and do a swipe on Swipe to wipe option
Step 5: Once the process is complete, install the files that you downloaded earlier. Select Install and then head over to system file and select it; now tap on Add more zip option. Now add all three files one after one. Do a swipe on the Swipe to confirm flash option at the bottom of the screen
Step 6: When the installation is complete, return to the recovery menu and select Reboot system
That's it. Your G4 should now be running stock Android Marshmallow. To know more about the TWRP-flashable stock ROM, head over to XDA development thread.