That's the AT&T OTA update app. Just let it update your phone and it will go away. Sometimes, there are small updates to fix some minor bugs, not necessary related to Android OS update.
You can't downgrade firmware without going through hell:
* you need find a copy of old firmware (check samfirmware.com) that targets your specific model. Samsung makes gazillion variants of the same phone. So make sure you find the correct one.
* you will have to manually put your phone into download mode.
* You need download ODIN3 on your PC and use it to push firmware to your phone that is already in download mode.
* Pray the process won't brick your phone (yes, it happens).
* It will wipe out everything on your phone (your settings, data, photo and apps).
OTA updates don't have any of these hassles but you can only update what it currently offered by the server. There is no way to go back.
Even if you downgraded, you will still face the same update prompt because that's how AT&T force your phone to do.