AT&T has started rolling out a new update to Samsung Galaxy Note5 units on its network. The update, which bumps the build number to MMB29K. N920AUCS2BPG1 and weighs in at around 23MB, brings along the Android security patch for the month of July.
Keep in mind that you must be on current software version (ending in PE6) in order to receive the update.
As is usually the case with OTA updates, it may take some time before the update notification pops up on your device's screen. Meanwhile, if you feel impatient, you can manually check for the update by heading to your handset's Settings menu.