This is basically just small OTA update, as each device update only clocks at 17 MB in file size. So it wouldn't take too long to update the device at a right time.
What's new: Android Device Security Updates - August 1, 2017
AT&T's Galaxy S6 Update version G920AUCS6EQH1
- Device: Galaxy S6 SM-G920V
- Android version: 7.0.1
- Security patch level (SPL): August 1, 2017
- Baseband version: G920AUCS6EQH1
- Kernel version: 3.10.61-10950877
- Build number: NRD90M.G920AUCS6EQH1
- Build fingerprint: 7.0/NRD90M/G920AUCS6EQH1
AT&T's Galaxy S6 edge Update version G925AUCS6EQH1
- Device: Galaxy S6 edge SM-G925V
- Android version: 7.0.1
- Security patch level (SPL): August 1, 2017
- Baseband version: G925AUCS6EQH1
- Kernel version: 3.10.61-10950877
- Build number: NRD90M.G925AUCS6EQH1
- Build fingerprint: 7.0/NRD90M/G925AUCS6EQH1
AT&T's Galaxy S6 edge+ Update version G928AUCS4EQH1
- Device: Galaxy S6 edge SM-G928A
- Android version: 7.0
- Security patch level (SPL): August 1, 2017
- Baseband version: G928AUCS4EQH1
- Kernel version: 3.10.61-10809541
- Build number: NRD90M.G928AUCS4EQH1
AT&T's Galaxy S6 active Update version G890AUCS6DQH1
- Device: Galaxy S6 active SM-G890A
- Android version: 7.0.1
- Security patch level (SPL): August 1, 2017
- Baseband version: G890AUCS6DQH1
- Kernel version: 3.10.61-10950877
- Build number: NRD90M.G890AUCS6DQH1
- Build fingerprint: 7.0/NRD90M/G890AUCS6DQH1
So if you’ve got a AT&T Galaxy S6, S6 active, S6 edge, and S6 edge+, you can manually check for a software update by going into Settings > About device> Check for an Update.
How to update a Galaxy S6, S6 active, S6 edge, and S6 edge+ via Over-The-Air:
Ensure your battery is fully charged before starting the software update. This is a must and very important part to get rid of some issues if ever the software update being interrupted due to a power failure.
From the Home screen, tap the Menu Key > Settings > About device > Software updates > Check for Updates.
If your device finds a new software update, tap Download now. When complete, a screen will appear advising you that the new version of software is ready to be installed. Tap Install the update.
The device will power down and power back on. A screen will appear stating that the installation has been completed. Tap OK.
source: AT&T S6 (here), S6 edge (here), S6 active (here), S6 edge+ (here) support pages.