Connect your iPhone to your computer. Allow iTunes to launch on its own. If iTunes does not start on its own, open it manually by clicking on the desktop icon on your computer.
Hold down the "Power" and the "Home," the round button with the square on it, buttons at the same time until the phone turns off. This should take about 20 seconds. Wait until the screen goes completely dark.
Release the "Power" button and continue to hold down the "Home" button. Within about 10 seconds, iTunes should notify you that your iPhone has been recognized by the system with your iPhone appearing in the list of devices in the side menu panel.
Hold down the "Shift" and "Alt" buttons on your keyboard at the same time and press the "Restore" button in iTunes. Open the original firmware file by clicking on it, instructing iTunes to downgrade to this version.
Wait for the downgrade process to finish. You may receive an error message due to the fact that you have not allowed iTunes to complete the upgrade process as planned, but you can ignore it by pressing "OK." You should see a message on the iPhone's screen though, reading "Connect to iTunes" with a small yellow triangle next to it, which means the downgrade has been successful.
Turn off your iPhone and restart it before using it. This will confirm that your iPhone is working properly and will also notify AT&T to restore your original service to the phone.