HTC has already rolled out the Android 6.0 Marshmallow update to the One M8 Dual SIM, One A9 and One M9, but it hasn't come up with the firmware update schedule for other eligible devices so far.
However, leakster LlabTooFeR @LlabTooFeR has now revealed the Android 6.0.1 Marshmallow update will come to HTC Desire 816 within two weeks. The source later tweeted the firmware has come to the device in Turkey, but it's not confirmed yet.
Desire 816 (EU) to get Android 6.0.1 and Sense 7.0 within next 2 weeks. Stay tuned :)— LlabTooFeR (@LlabTooFeR) February 23, 2016
The same leakster had earlier claimed in a tweet HTC Desire Eye would get the Android 6.0.1 Marshmallow update with Sense 7.0 later this year, without revealing a tentative release date.
It may be noted that HTC's product management chief Mo Versi had tweeted earlier that the T-Mobile variant of HTC One M9 havd already received technical approval for the firmware OTA update on Feb. 16. It gave a hint that other major carriers might follow suit.
There were also reports that Sasktel, Rogers, Wind, and Videotron variants of HTC One M9 would get the Android 6.0 Marshmallow update soon.
HTC has confirmed several devices like HTC One M8, HTC M9+, HTC One E9, HTC ME, HTC One E8, HTC M8 EYE, HTC Butterfly 3, HTC Desire 826, HTC Desire 820, and HTC Desire 816 will receive Android 6.0 Marshmallow update.