HTC has already announce that they are currently working on the Android 6.0 Marshmallow upgrade for the HTC One M8 model and HTC One M9 model but when will the upgrades be arriving.
So far, HTC has only managed to upgrade the HTC One M8 Google Play Edition. HTC One M8 users will be glad to know that the HTC One M8 Android 6.0 Marshmallow upgrade will be available in less than 24 hours. This was announced on their twitter page so unlock HTC One M8 owners might want to keep checking for updates soon. M8 users will get the new HTC Sense UI 7 when the update arrives.
As for the HTC One M9 model, HTC did not say when the update will be arriving but they did say that they are still aiming for the HTC One M9 Android 6.0 Marshmallow update to arrive for the end of the year. There is still no official changelog yet so e will have to wait and see what else the update will bring to the table.