HTC is going to have a special announcement that we previously reported on. On April 12, HTC is expected to announce their newest flagship phone with the HTC One M10, or simply the HTC M10, or just the HTC 10. That will have Android version 6.0 (also known as Android M or Marshmallow) right out of the box, which means that all their other phones should have it as well. Here is the latest news about Android M Version 6.0 Marshmallow Release Date for HTC One M9, the One M8, and the Desire series for Verizon, Sprint, AT&T, US Cellular, and T-Mobile.
As far as previous flagship models are concerned, the HTC One M9 and the One M8 should be completely updated, at least according to Android Pit. So it looks like HTC has its flagship phones from two years ago taken care of.
If you are a user of one of these past flagship phones, check under "Settings" and "System Update" to see if it is available. It is possible that not all United States carriers (US Cellular, Verizon, AT&T, T-Mobile, and Sprint) have received it yet, as some carriers tend to get it before others.
Then there other HTC devices like the HTC One M9+, the HTC One E9+, the HTC One E9, the HTC One ME, the HTC One Butterfly 3, HTC Desire 826, HTC One 820, and HTC Desire 816. All of these devices are confirmed to receive the Android upgrade.
In fact, some HTC devices, such as the HTC One E8 and the HTC M8 EYE already have their updates rolling out. Users of these devices are also encouraged to check their "Settings" and "System Updates" to see if they have become available.
Sadly, there are some older HTC devices that will not be receiving the Marshmallow upgrade, and this usually applies to devices that are over two years old. This includes the flagship HTC One M7, the HTC One Mini, and the HTC One Mini 2, and there is no point for any user of any carrier of these devices in any location to check to see if an update is available.
One good source for the HTC updates is Mo Versi, the VP of Product Management at HTC. He often sends out tweets concerning the latest HTC updates, and he's sort of the company's unofficial spokesperson of this, assuming that he isn't the official spokesperson. One of his most recent posts discusses the #Powerof10, the newest HTC phone that will be seen within the next two weeks.