T-Mobile has released a guide on when to expect the Android 6.0 Marshmallow update for devices on its network. Some Android smartphones sold by T-Mobile have already received the update as of now, while most of the others are in the development process.
Here's the list and a link to T-Mobile's website where you can find a complete list of devices...
T-Mobile says the Marshmallow update roll out will happen in three stages. Currently, most smartphones listed below are in the "Manufacturer Development" stage or Stage 1. Stage 2 is "T-Mobile Testing" while Stage 3 is the final stage where testing is complete and the update gets released.
This is the explanation from the carrier...
Software updates are a long process, which require a partnership between the software developer, the device manufacturer, and T-Mobile. After initial software is developed, the device manufacturer works with T-Mobile to customize the software, including changes for your device hardware, the T-Mobile wireless radios, and applications.Once the update is available your device will provide a notification that the update is available with direction on how to download it. You do not need to contact T-Mobile or the manufacturer. Customer Service uses the same information that customers can access online.
This list includes the stage at which each device is, according to T-Mobile (not all devices are listed)...
HTC One M8 - Manufacturer Development
HTC One M9 - Manufacturer Development
LG G3 - Manufacturer Development
LG G4 - Manufacturer Development
LG G Stylo - Manufacturer Development
Google Nexus 5 - Now Available
Google Nexus 6 - Now Available
Google Nexus 7 - Now Available
Google Nexus 9 - Now Available
Samsung Galaxy Note 4 - Manufacturer Development
Samsung Galaxy Note 5 - Manufacturer Development
Samsung Galaxy Note Edge - Manufacturer Development
Samsung Galaxy S5 - Manufacturer Development
Samsung Galaxy S6 - Manufacturer Development
Samsung Galaxy S6 Edge - Manufacturer Development
Samsung Galaxy S6 Edge+ - Manufacturer Development
Samsung Galaxy Tab S2 - Manufacturer Development
You can check out the T-Mobile website for complete details.