If you're using an on , your wait for is almost over. HTC's Mo Versi has confirmed that the update will start rolling out on Tuesday, February 16.
Happy Friday! Good news for T-Mobile HTC One (M9) owners! We've received technical approval on Marshmallow, with the OTA to start on 2/16.
— Mo Versi (@moversi)
HTC originally rolled out the Marshmallow update to the unlocked HTC One M9 back in December, and it started landing for carrier variants in January. T-Mobile will be the second carrier in the U.S. to get the update, coming just behind Sprint, which began it's rollout .
Source:
Central
HTC One M9 Essentials