It's not often the case, but T-Mobile is playing catch-up with some other carriers when it comes to getting OS updates to a flagship phone. Sprint M9 owners got it on the first of July and AT&T versions were updated two weeks ago, but at not-so-long last T-Mobile HTC One M9 owners will be upgraded to Android 5.1 as well.
Of course, the T-Mobile M9 got the oft-mentioned camera improvement update way back in April, so they were first to get what was arguably the most important OTA for the M9.
The update, to version number 2.7.531.6, should start rolling out today. You can wait for a notification or just manually check for the update and it should be ready, according to T-Mobile.
Much-wanted battery improvements are promised as part of the update along with factory reset protection. You must have a screen lock of some kind to benefit from factory reset protection, referred to as "anti-theft" in T-Mobile's changelog. Google Wallet will also become a pre-installed app, which is stretching the definition of what a "new feature" really is.
The download is 833MB, so be sure to have a quick, non-metered internet connection before starting the download.
Full RUU Download
If you've been waiting for the full RUU, it's here. It's a hefty 2.15GB download - grab it from here.