Update: It's available now!!!
HTC has announced that owners of the unlocked HTC One M8 will begin seeing the Marshmallow update hitting their phones very soon. Mo Versi, HTC's VP of Product Management sent out a tweet that said that testing has been completed, and the update will begin pushing out within 24 hours. The Marshmallow update brings a variety of improvements to the One M8, like Doze, enhanced permission controls and much more.
HTC One (M8) unlocked owners! Great news, all testing has been complete and the update for Marshmallow will be sent out within 24 hours. Thx— Mo Versi (@moversi) December 2, 2015
If you own an HTC One M8, you'll want to be on the lookout for the update. To check manually, head into your Settings, then About phone and check for the update. Be sure to let us know when you begin receiving it, and how the update works out for you.
Source: Mo Versi (Twitter)
Newer model: HTC One M9