Samsung's latest round of beta tests for Android 6.0 Marshmallow are now showing up. This one is for the AT&T version of the Galaxy Note 5. Only around 200 people are being included in this initial testing round, so it could still be some time before we see a wide rollout.
Those lucky few selected for the test will see update MMB29K.N920AUCU2BPB1. Of course, the final version after the test will likely have a different number.
Source: SamMobile