The update is available over the air with the latest software build H81120i, and it weighs in at around 963MB. The only requirements for downloading the update are a data connection (obviously), 50% or higher battery life (that’s reasonable) and 963MB of free space. If you haven’t yet received a notification, you can check right now by opening the settings app, and navigating through About Phone –> Update Center –> System Updates –> Check for Update.
The update should hit your handset over the air soon if it hasn’t already be cool and If you want to force the update you should head over the T-Mobile website and download the Software from there and follow the instructions over there. If any one of you have updated your LG G4 to Android Marshmallow than let us know what were your experience via comments.