A profile is a background software that AT&T put into an iPhone to tell it how to run. It's not the firmware within the phone, but it tells the firmware how to run. You cannot remove the base profile that AT&T assigns the phone as it needs it to run. You can, however, download a hacked profile if you want to tether (feed iPhone Internet to your computer) your iPhone. This hack has been known to cause problems, though, and you can remove this profile or any other one that replaced the AT&T profile easily.
Navigate to the "Settings" menu on the iPhone, then move to the "General" menu. Access the "Profile" menu to see what profile is currently installed over the original AT&T profile.
Tap on the red word "Remove." When asked if you want to remove the profile, click "Yes" and go back to the home screen.
Restart the iPhone so that it can roll back to the original AT&T profile. The previous iPhone profile is deleted when you power on the iPhone again.