Following up the recent release of Android 6.0 to unlocked G4 that was sold across Europe, South Korea, US Cellular and Canada’s Telus and Rogers, LG again made another Marshmallow available on T-Mobile LG G4 via PC synchronization software LG Bridge.
Number of users reported able to update their G4 to Marshmallow by simply connecting their device via USB to PC and then by checking for update manually. Note that over the air update is still not yet available and T-Mobile’s support page for G4 does not officially mention the availability of the said update.
As other LG devices getting Marshmallow upgrade, you are looking at improvements for Knock Code, the recent implementation of new app permissions model, redesigned Silent Mode, Peek mode, and visual changes.
So if you are into rooting and ROM-ing, there are plenty of XDA-Developers in the web that already have covered with fully stock ROM available for the devices just by flashing, as well as the necessary rooting methods.