It's AT&T's turn to update the rear camera on the HTC One M9
The new software version will be 1.32.502.31, and HTC suggests that you hold 20 minutes aside for the process of updating your One M9. The manufacturer also recommends that you recharge your battery to at least 35% and have your phone plugged in while the update is in progress. Do not disconnect your phone until the entire update is completed or you might brick the device! We suggest being connected to a Wi-Fi network when updating your phone since the entire process runs about 20 minutes.
If your phone has yet to receive the notification, you can sync your HTC One M9 to a PC and download the update directly from the HTC Sync Manager. Updating your phone manually means wiping all data from it and flashing a 2.1GB file. If you decide to go this route, back up your data before you start.
Now that AT&T has started rolling out this update, customers of all four major U.S. carriers with the HTC One M9 have the same software update inside as those with the international version of the phone.
source: HTC via AndroidCentral