Some time ago Google officially revealed Android 6.0 Marshmallow, their next generation of Android operating system. Android 6.0 brings lots of features, improvements and fixes over Android 5.0 Lollipop. You can read all about Android M, including new features and detailed changelog by clicking here.
Now Android 6.0 Marshmallow roll out has been started for Nexus 6 users on T-Mobile. However it is worth to note that not all Nexus 6 devices will get the Android 6.0 update at once, it will start showing up on your device very soon.
T-Mobile has also provided more details of Android 6.0 update on Nexus 6, you can check it by clicking here