The HTC One M9 has already received the update to Android 5.1 Lollipop - be it the developer version, or those sold by both Sprint and AT&T. This hasn't, however, been the case so far for T-Mobile's model of HTC's latest flagship smartphone.
Until today. The magenta carrier has announced that the new software is finally rolling out to the handsets it's sold. As always with such over-the-air updates, it may be a few days (or even weeks) before all One M9 units purchased from T-Mobile get the download notification, but at least it's on its way now.
Aside from the platform upgrade to Android 5.1, the new release brings with it some battery improvements, Google Wallet, as well as anti-theft functionality built into the OS. After applying the update, you'll be running software version 2.7.531.6.
To apply the 833MB update, you need to have at least 50% or higher battery left, and use a Wi-Fi connection for the download.