Recently I brought a used Korean version LG G3 mobile phone for a Virtual Reality (VR) project. The mobile phone required update to the latest version of Android operating system. This Korean model of LG G3 didn't offer Over the Air (OTA) updates.
The first step in the update process is to connect the model phone to the computer using USB cable. However the standard Android 'Google USB Drivers' didn't work. After some searching I found that LG Universal Mobile Drivers from LG support site.
Next I used the LG Mobile Support Tool program on my computer to start updates. This model is not supported by LG Bridge Software. After couple of updates I managed to upgrade the mobile phone to Android 6.0 Marshmellow. Each update requires a download of 1GB update and you need to wait for the phone to reboot and optimize all the Android applications.
There was one glitch in the upgrade. Had to restart an upgrade when I faced 'Upgrade stopped due to an error' message.
Here rather straight forward way to upgrade the phone to latest Android 6.0 (Marshmellow):