When it comes to the latest version of Android, it has taken months before it has come to the latest flagship phones, including HTC. Fortunately, HTC is updating some of their newer Android phones with Android M (also known as version 6.0 or just Marshmallow), including the HTC One M9 and HTC One M8, among others. There is also news about the latest flagship phone the HTC One M10, which could also be known as the HTC 10.
Much of what is known about the next flagship phone from HTC is that it will be called the HTC 10, rather than the next logical name in the series with the HTC One M10. As far as the features go, Ubergizmo reports that it will have a 5.1 inch QHD display, a Snapdragon 820 chipset with 4GB of RAM, a 12 Megapixel rear-facing camera, a USB Type-C port, and possibly the front-facing BoomSound speaker setup of its predecessor.
It is also thought that the HTC 10 will have some color variations of black, gold, white, and silver, and it will have an all metal back. As far as the release date goes, many reports are pointing to April 19 in New York, and it could possibly be released on that date, according to Ars Technica.
It is pretty obvious that that HTC 10 will come with Android Marshmallow right out of the box. As for the older phones, HTC has a good reputation for setting a target date for upgrading their newer Android phones 90 days after the initial upgrade release date of Android (assuming the device is less than two years old).
There has been a few times where HTC has missed that time, and it was promised that the M9 and the M8 were supposed to have Android 6.0 by the end of last year. Some carriers actually made that date, but others did not. Then again, it is common for major U.S. carriers like US Cellular, Verizon, T-Mobile, AT&T, or Sprint to receive the upgrade of Android beforeothers.
Considering that the Android Marshmallow upgrade has already come to the HTC One M9 already, at least for T-Mobile, Verizon, but not AT&T, it should roll out to all planned HTC phones listed below. There are already reports of Android M coming to the HTC One M8 in the UK. There is a recent report from Gotta Be Mobile saying that version 6.0 has come to Verizon Wireless as well as T-Mobile.
In addition to those flagship phones from HTC, these other phones are planning to get the Android upgrade: HTC Butterfly 3, HTC Desire 816, HTC Desire 820, HTC Desire 826, HTC One M9+, HTC One E8, HTC One M8 EYE, HTC One ME and HTC One E9.