Downgrade iOS 5.1 using iFaith
AppleiPhoneTechUK gives instructions as to how to downgrade IOS 5.1 to a lower firmware using iFaith.
Firmware links:
TinyUmbrella: http://thefirmwareumbrella.blogspot.co.uk/
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iFaith for windows: http://downloads.ih8sn0w.com/iFaith-v1.4.2_windows.zip .