Use the following steps to restore your device:
Make sure you have the latest version of iTunes installed and running before attempting to update.
Connect your device to your computer.
Select your iPhone, iPad, or iPod touch when it appears in iTunes under Devices.
Select the Summary tab.
Select the Restore option.
When prompted to back up your settings before restoring, select the Back Up option (as seen in the image below). If you have just backed up the device, it is not necessary to create another.
Select the Restore option when iTunes prompts you (as long as you've backed up your device, you should not have to worry about restoring your device).