HTC is one of the companies that seems more interested in updating their devices to the newest versions of the Android operating system. They were among the first ones to publish a list of the devices that would get updated to Android 6.0 Marshmallow and they even released the One A9 with this version of the OS out of the box, this phone is one of the few devices currently running on Marshmallow. HTC is working on bringing the update to some other devices such as the One M8 quite soon, and now that Android 6.0.1 has been announced, the company communicated that they will update some phones to this new version starting this month.
LlabTooFer has become known for providing some leaks which have turned out to be true, and now that user has published a picture with the alleged schedule for some HTC devices to get updated to newer versions of Android. Basically, every mid-range or high-end device from 2014 and 2015 will get updated to Marshmallow with HTC's custom skin called Sense 7.0. He also mentions that Dual SIM versions of the devices in the picture are getting the update as well, although those might take a few extra months before they run the new software. Finally, he makes it clear that HTC could change their plans along the way, so the update might get cancelled or postponed for some devices. Even though the leakster has a certain confidence level, this is still not officially confirmed at this point.
The One M7 will no longer be supported by HTC, so it will remain running Android 5.0.2 with Sense 6.0, although it is possible that some security updates might get released for this phone. The One A9 is listed to receive Android 6.0.1 in early Q1 2016, while the One M9 will be updated to Android 6.0 in late December 2015 and Android 6.0.1 is scheduled for Q1 2016. Some devices will skip the Android 5.1 update and will get updated to Android 6.0, those include the One M9+, One ME, One M9+ Supreme, One (M8), One (E8) and the One M8s, the last phone will get updated on Q2 2016, while the rest will get the update in Q1 2016, the company is evaluating the update to Android 6.0.1.
Moving into the mid-range, the Butterfly 3 will apparently skip the Android 6.0 update and get updated to Android 6.0.1 in Q1 2016 while the Butterfly 2, Desire 826 and Desire 820 will skip Android 5.1 and 6.0 and is scheduled to receive the Android 6.0.1 update in Q2 2016. The Desire EYE is running Android 5.1 in Japan and the company is still evaluating if it will get updated to Android 6.0.1. The Desire 816 will skip Android 5.1 and will get updated to Android 6.0 in Q2 2016, while the Android 6.0.1 update is still in evaluation. The Desire 626 will get updated directly to Android 6.0.1 in Q2 2016 and the Desire 526 is scheduled to receive Android 6.0 in Q2 2016 while the Android 6.0.1 is being evaluated.