Although it arrived back in 2015, the One M9 is still a capable flagship, especially since it receives Android Nougat via the official channels. AT&T users still using this HTC handset should get the new firmware in a few days, since the official update page is already up.
https://www.notebookcheck.net/Android-7-0-Nougat-coming-soon-to-HTC-One-M9-on-AT-T.221806.0.html