What you need:
Install LG UP TOOL
Install LG USB drivers
Install LG UP DLL for LG MS631
Download: LG MS631 stock firmware: MS63120b.
The LG UP DLL for LG MS631: LGUP_MS631.msi is essential for LG UP to detect your phone model and software Version.
How to Manually Install Android 6.0 Marshmallow on MetroPCS LG G STYLO MS631 with LG UP
Step 1) Connect Your LG G Stylo MS631 to the computer using USB cable. Ensure that LG USB Drivers are installed on the PC, so that the computer can find your device.
Step 2) Run the LG UP, If you the LG UP can find the LG MS631. Go Step 5
Step 3) If LG UP can’t find your device, you need to put your phone into download mode manually by
Turn off your phone, press and hold “Volume Up” button and plug in the USB cable from your computer to the phone.
Step 4) Now run the LG UP, waiting for LG UP install necessary USB drivers for your Phone. When LG UP screen shows up. Go Step 5
From LG UP Screen, Select Upgrade Or Refurbish
Click Bin File, Select the MS63120b kdz file you just download
And Then Click start.
Wait until the step shows 100% complete, unplug USB cable, and reboot your phone.
Notes,. Do not use KingRoot to root a LG G Stylo that runs on Android 6.0 M – Ver: MS63120b. It may cause boot issues.