HTC is making good on a promise it made earlier this month. Following that up, HTC's Vice President of Product Management, Mo Versi, announced the news on Twitter yesterday, saying the updates were being finalized for distribution in the next day (which is today). In addition, owners of HTC's unlocked One A9 handsets could expect an incremental bump to Android 6.0.1, which comes with extra emojis.
Happy Holidays! Updates to the Unlocked A9 6.0.1 and Unlocked M9 6.0 will be able to be downloaded within next 24 hours! Enjoy :-)— Mo Versi (@moversi) December 23, 2015
As for carrier versions of the One M9, Marshmallow is expected to arrive in early 2016. It takes a little longer for carrier versions because of the additional testing that's required, in part because of carrier specific tweaks.
The One M9 isn't the newest kid on the block, though it's still a decent flagship. You can read our full review for an expanded impression.