AT&T updates the HTC One (M8) and HTC One M9 with security patch to combat Stagefright
When you start the process of downloading and installing the update, AT&T demands that you be connected to a Wi-Fi network in order to receive it. At the same time, you should have your battery charged to at least 50%.
Because the update is being sent OTA, it might take a few days before it actually arrives on your phone. You can try to manually call for the update by going to Settings > AT&T Software update > Check for updates. The HTC One M9 is also receiving a similar update, so if you are rockin' that device, keep an eye out for the notification that your phone has received it. The update for HTC's current flagship phone weighs in at 55MB and contains software version 2.6.502.18. You need to be on a Wi-Fi connection to receive the update.
You might think that because the update doesn't add any cool new features that you can actually see with your eyes, it isn't important to download and install. But considering how the Stagefright exploit can give control of parts of your phone to someone you don't know, it is truly important to prevent this from happening.
source: AT&T (1), (2) via AndroidCentral