1-Plug the USB cable into the iPhone to disable. Connect the other end to a USB port on the computer with which you last synced it.
2 -Open iTunes if it does not open automatically.
3-Click iPhone Devices section on the left side of the iTunes window. The Summary page appears.
4 -Click the "Restore" button in the center of the Summary page. A dialog box will appear.
5 -Click on the backup device. In the next dialog box, click "Restore." Leave your iPhone connected at all times.
6 -Click to set up your iPhone as a new phone or a backup when the restore is complete and a new page appears in iTunes. If you choose to use a backup, click the backup date from the drop down menu. If you choose to set it as new, enter the required information and select the media you want to sync.