HTC's seasonal flagship, the One M9, shipped with Android 5.0 Lollipop out of the box, but in the meantime Google released another update to its venerable mobile OS. Samsung already started rolling out the Android 5.1 update for its Galaxy S5 and S6 handsets, at least on T-Mobile, and HTC will be next in line, it seems. Famed developer LlabtooFeR already outed a custom ROM with the latest 2.7.401.1 firmware that will carry Android 5.1 to your One M9.
If you don't want to wait for the update to hit your handset, you can install this custom ROM, but you need to be rooted, with recovery installed, and so on. What the latest Android will bring to the M9, besides the obligatory bug squashing, is a new native display color management, and multi-user support. The DND mode, and alarm control settings have found a comfortable placement next to the volume slider in the notification bar, too. Head down to the source link if you are interested in the instructions how to install the Maximus HD custom Android 5.1 ROM for your HTC One M9.