To restore an iPhone with iTunes, open iTunes, click Help, and click Check for Updates to make sure you have the latest version of iTunes. Connect the iPhone to the computer via the USB cable that came with the iPhone, and then select your iPhone under the Devices menu.
On the Summary tab, click the Back Up Now button if you want to back up your current files, photos, contacts and applications. Click Restore iPhone, click Restore again when the confirmation window appears, and then wait for the restoration process to complete. The Slide To Set Up screen appears when the restoration process ends. Select Set Up As New iPhone if you want to set the iPhone as new, or select Restore From iTunes Backup to choose a previous back up. If you choose to restore the iPhone as a new device, follow the on-screen instructions provided by the Setup Assistant.
Alternatively, connect the iPhone to the computer via the USB cable, open iTunes, click on the name of your iPhone under the Devices menu, and then select Restore From Backup on the shortcut menu. Select the specific backup, and then click the Restore button. If the device had an active cellular service before restoration, the cellular service activates when the setup process ends.