Earlier today, Mo Versi tweeted that HTC One M9 and One A9 are about to get Android 6.0 and Android 6.0.1 OTA respectively. It seems like it is happening now as HTC One M9 users over XDA have already started receving OTA update notifications for Android 6.0 Marshmallow with update serial numbers 2.11.617.180 and 3.35.617.12. Actually, the process consists of two updates.
The first update 2.11.617.180 prepares your phone for the Android 6.0 installation process and the second update 3.35.617.12 actually installs Android 6.0. First update is a small one and weighs only around 99MB, whereas the second update weighs 1.21GB. Along with Android 6.0, Marshmallow, this update brings improved app permissions, Now on Tap feature, Doze mode and much more.
If you have not received any update notification on your Unlocked HTC One M9 yet, then you might want to check for updates from your device settings.