The HTC devices which are confirmed to get the Android Marshmallow upgrade are HTC Butterfly 3, HTC Butterfly 2, HTC One M9, HTC One M9 Plus, HTC One ME, HTC One E8, HTC One E9 Plus, HTC One M8 Plus, HTC Desire Eye, HTC Desire 816, HTC Desire 820, HTC Desire 826, HTC One E9 Plus, and HTC One E9.
However, this list is a preliminary list for the moment, and it may get more additions in the future. Twitter handle LlabTooFer recently confirmed this fact in a tweet.
Android Marshmallow is most likely to arrive on Sept. 29, alongside the release of Google Nexus 5X. The Nexus devices are expected to be updated with Android Marshmallow at the beginning followed by Samsung devices. The exact release date for these HTC devices is not yet known.
The Android Marshmallow upgrade will come in the form of an over the air update. This implies that different devices will get the update at different times. The notification of the software update will be seen at the top left corner of the screen. If the users want to check for the update manually, they can go to Settings -> About Device -> Check for Updates
Android Marshmallow is expected to bring a lot of new changes and improvements in the devices, according to BGR. It will bring the support for the finger print scanner, Bluetooth 4.2 connectivity, better Wi-Fi connectivity, a new API level 23 for more enhanced animations, support for USB Type C connector, support for 4G LTE connectivity, and many more. It will also provide support for Android Pay.