Good grief, T-Mobile has been awfully late in providing the Android 6.0 update to Nexus 6 users on its network. That kind of behavior is positively... carrier-like. But as of Friday the rollout for Marshmallow should have finally begun, at least if you're anywhere near a Wi-Fi network - according to T-Mobile's support document, a full OTA including the cellular network will begin later. As always, it might be several days until you see the update alert on your specific device.

t-mo

The build mentioned on the support page is MRA58X, which currently isn't available on Google's Nexus image download site. That's probably because T-Mobile uses a couple of software features that aren't necessary on other carriers, particularly support for LTE band 12 and T-Mo's specific implementation of Wi-Fi calling (which hit Lollipop back in May). Google is currently hosting older Lollipop builds for T-Mobile, so the download page will probably be updated at some point for those who want to manually flash Marshmallow.

T-Mobile's support page doesn't mention any changes aside from the Android 6.0 upgrade, but that's not unexpected since the Nexus 6 runs on (nearly) AOSP software builds. The OTA upgrade file size is just under 700MB, and you'll need a 50% charge and a Wi-Fi connection to download it.