Software version G920TUVU3EPD1, and G925TUVU3EPD1 are pushing out to two of T-Mobile's devices. The Samsung Galaxy S6 and the S6 Edge are the lucky recipients (respectively). These updates aren't just minor tweaks and fixes either. These are the full on Android 6.0.1 Marshmallow experience. We don't have a changelog, but the usual features which come standard with Marshmallow are on tap.
For those who can't wait to get the OTA push, here's a method to try and force it: hit Settings > About phone, tap the 'Check for updates' repeatedly. Here's our dedicated section for further discussion of the Galaxy S6 and S6 Edge: Samsung Galaxy S6, S6 Edge and S6 Edge Plus