AT&T is finally sending out the update to Android 7.0 Nougat for the HTC One M9 units it's sold. The unlocked version of the phone got the same software back in December of last year, so this has definitely been a long time coming. More recently, we've seen T-Mobile rolling out the update in March.
Now though it's heading out to One M9 devices purchased from AT&T. Since it's an over-the-air rollout, expect a few days to pass before every single unit out there receives the update notification. If you're impatient, however, there's always a chance you'll get it faster if you manually check for it by going to Settings > About phone > Software updates.
Keep in mind that you must be on Wi-Fi to grab the 1.4GB download. After installing it, you'll be on software version 4.27.502.7. And, sadly, you'll only have the February 1, 2017 security patch level - so you'll be missing the fixes that have been applied in subsequent security rollups in March, April, and May.