The Android 6.0 M software update is available for T-mobile USA LG G3 D851. If you can not get the update via OTA or LG bridge, you can use below guide to manually install the Android 6.0 M Software on your LG G3 D851 phone.
What You need:
LG USB drivers Ver 4.0.4
LG UP Store Frame
LG UP DLL for G3 [msm8974 soc]
LG G3 D851 Android 6.0 Firmware: D85130d_00_0404.kdz
How to Flash Android 6.0 M stock firmware on LG G3 D851 using LG UP:
Step 1, Install the USB Drivers
Step 2, Install the LG UP Software – LGUP_Install_Ver_1_14_3.msi
Step 3, Install the LG 8974 DLL — LGUP_8974_DLL_Ver_0_5.MSI
Step 4, Now connect your Phone to PC Via USB Cable and Run the LG UP Software
Wait until LG UP detect your Phone. If LG UP screen shows up, Go Step 5.
If LG UP can’t find your device, you need to put your phone into download mode manually
Turn off your phone, press and hold “Volume Up” button and plug in the USB cable from your computer to the phone.
Now run the LG UP, wait LG UP install necessary usb drivers for your Phone.
When LG UP screen shows up. Go Step 5
Step 5, From LG UP Screen, Process option, Select Upgrade
Click the 3 dots shown in above screen shoot,, Select the KDZ file you just download
And Then Click start.
Wait until the step shows 100% complete, unplug usb cable, and reboot your phone,
You should have Android 6.0 M now on your LG G3 D851 Phone