We have multiple reports that HTC One M8 owners in the U.S. have already started receiving the Android 6.0 Marshmallow update. HTC had promised to try and get the update pushed out to M8 owners by the end of the year, and it looks like they made good on that promise.
There is one caveat... this version (6.12.1540.4) is only pushing out to unlocked and Developer edition HTC One M8 units in the US. There's no guarantee that the update will hit any carrier branded HTC One M8 units before 2016.
For further discussion, head over to our dedicated HTC One M8 section here: HTC One (M8) | Android Forum at DroidForums.net