If you're on AT&T with the HTC One M9, you'll be happy to know that Android 5.1 is rolling out starting, now. We heard earlier this week that AT&T had approved the update and would be rolling it out later this week. And later has come. You can head into the Settings, tap on About Device, then System Updates and Check for updates to claim your Android 5.1 goodness. However, if that doesn't pull down the update for you - remember AT&T only allows you to check for an update once in a 24-hour period - you can use the RUU and flash the update to your HTC One M9 yourself. You can find that in the source links below. However, if you are not comfortable with flashing the RUU, we highly recommend you wait until the update is available to you.
The two major pieces or new features in this update is Android 5.1 Lollipop, and Device Protection. With Android 5.1, it's not a huge update from Android 5.0, but it is an update you'll want. With Android 5.0, there were quite a few bugs. Which is quite normal when there's a huge update to the OS like Android 5.0 was. And Android 5.1 fixes the most of those bugs and makes Lollipop much more stable to use on the HTC One M9 and other devices that are running the update already. One of the big features of Android 5.1 is Dual-SIM support, which won't matter for the HTC One M9 on AT&T, however.
For those of you wondering, the build number for this update is 2.6.502.16. AT&T managed to get their Android 5.1 update out before Verizon and T-Mobile, being just behind the developer edition and the Sprint versions. Sprint is usually pretty fast at approving updates, so that shouldn't be a huge surprise. It is definitely nice that HTC and AT&T published the RUU, so those that don't have the patience to wait for the update to hit their device can go ahead and flash the update right now. This update should make your HTC One M9 run a bit more smoothly, always a nice thing to have after an update.