Google the 3.1.3 os and download it. Connect your ipod touch to itunes via usb cable, (backup your ipod before beginning), hold down the "alt" key and click restore, this should bring up a window so you can select which file to use to restore the ipod. Select the ipsw (os file) you downloaded and click ok. It should now restore it up to the 3.1.3 you downloaded.
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