The HTC One M9 is one of the most powerful devices out there, it combines a great design made of a dual-toned once piece of aluminum and some of the best internals. The software experience is also considered among the best ones thanks to the company's customization of the Android operating system called Sense in its seventh version. This version was based on Android Lollipop, although not many changes were spotted compared to its previous version. The latest version of the OS is Android 5.1 which is a small update, but it fixes most of the problems that were found on the first release of Lollipop.
HTC has been updating this particular model to the latest version of the OS. As it is usual, first it was released for the Developer Edition of the One M9, which features an unlocked bootloader and no additional software of the carriers. Some time afterward, the update started appearing for some carriers in the US, AT&T and Sprint, leaving T-Mobile and Verizon in line to get it. Today, the update is being released for the T-Mobile version of the phone, a little later than expected. If you are one of these users you can check for the update going into Settings, in the Phone section tap About, select Software Updates and Check Now. The most recent version is the 2.7.531.6, which may also be prompted over the air (OTA).
The official changelog provided by T-Mobile states that the update includes Android 5.1, Anti-theft, and Google Wallet while Battery Improvements will be integrated into this update as well. When Anti-theft settings are turned on, it prevents users to reset the phone to its factory status if the Google account is changed. Using a Wi-Fi connection is recommended as the update weights over 800 MB. Some other reported changes brought by the update are better performance for the camera, providing better colors in extreme situations, an enhanced exposure using the automatic settings and a reduction of digital noise, plus it is said to include better temperature management for the CPU, preventing it to overheat due to the controversial Snapdragon 810 that powers the phone.