As originally promised, HTC will bring Android Marshmallow to all of its flagship handsets, going back to and including the One (M8). Today the OTA has started seeding for both the Verizon and T-Mobile carrier versions of the phone.
The former is receiving a software version 6.21.605.3 package, while the latter has the build 6.20.531.5. Both, however are based on Android 6.0 and bring about all the Marshmallow goodies Google's OS has to offer, like run time permissions, app Dozing, Google Now on Tap and many others. The software should also come with new volume controls for notifications, audio and alarms and simplified screen capture.
In other news, T-Mobile is also spreading some Marshmallow love to its more price-conscious clients, owners of the LG G Stylo. It has also started receiving an Android 6.0 OTA, already spotted in the wild with a version number of "H63120b" and weighing in at 767MB.
Bear in mind that both carriers are conducting a staged rollout of all the updates, so it might be a few days before your handset receives the payload. T-Mobile has even cited a rather lengthy update window, ranging from March 7 to March 21, so a bit more patience might still be required.
I'm From Indonesia, my htc one m8 got the Marsmallow abaout 19 hours ago
I got a reminder on my M8 to upload android 6 which I accepted and 4 hours later
Hi troll kiddo. If you feel hatred towards HTC, please keep your mouth shut! Please read Reddit, XDA and see for yourself how freaking smooth and great the battery life is after MM update.