LG G4 receives Android 6.0 update over-the-air on T-Mobile
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. You will, of course, need to ensure that you're not running low on battery before proceeding, and also, connecting to a Wi-Fi hotspot would be wise unless you'd like to eat a gigabyte of cellular data.
Many will have already gone ahead and installed via Bridge, but if you haven't made the jump, the OTA requires very little effort. Originally slated slated to begin trickling out from February 10th, it's good to see that despite slow-moving nature of the wider Marshmallow release, T-Mobile exceeding its schedule.