After the update was rolled out to the HTC One M9 on Sprint and T-Mobile, it was inevitable that the better camera software would come to users on the bigger carriers. Starting today, the OTA camera update is now available on AT&T.
Keep in mind that many sites that got their hands on review units received international models, so your camera, if you are having issues, may not get as improved as you like.
The software version is 1.32.502.31 and HTC is also providing an RUU download for users who want to flash the update themselves instead of waiting for it to appear OTA. So if you want to download the update, head to the source link.