Jailbreaking and Firmware

• When you jailbreak an iPhone, the device becomes able to accept any carrier's SIM card, which increases your flexibility of using it. Not all side effects of jailbreaking are positive, however. In addition to the fact that jailbreaking voids an iPhone's Apple warranty -- and violates AT&T Terms of Service -- you can't update the iOS operating system or carrier software, collectively known as the device's "firmware," once you've jailbroken the iPhone. This prevents you from installing updates, some of which are critical to the proper function of the iPhone.
  
  

Restoring and Firmware

• In order to restore an iPhone successfully, you must have the latest version of the iOS operating system and the iPhone's carrier software installed on the phone. As a result, attempting to restore a jailbroken phone fails, unless you re-lock the phone to its original carrier, which is AT&T in the U.S. To do this, plug the iPhone into your computer and wait for iTunes to open, the click "Yes" when iTunes asks you to install software updates. Once you do this, the device no longer works with the secondary carrier, but is ready to be restored to its factory settings.
  
  

Restoring and Backup

• Each time you sync an iPhone with a computer, iTunes backs up the data on the iPhone and transfers new data onto it. Nevertheless, when you click the "Restore" button to begin the restore process, iTunes promote you to back the device up again. Choose "Yes" to have the peace of mind that your data is backed up without a doubt. After the restore process begins, the iPhone powers off and restarts, indicated by an "Apple" logo appearing on its screen.
  
  

Replacing Data

• Once the iPhone restarts, iTunes prompts you to replace data onto the phone and displays an interface with a drop-down menu that contains the dates of the most-recent backups. The process of getting data back onto the phone can be short or long, depending on how much data you need to transfer back onto it. If iTunes doesn't automatically prompt you to replace the iPhone's data, right-click on the iPhone icon in the left navigation pane and select "Restore From Backup" to access the menu.