Cannot Update to iOS 5.1

i've tried repeatedly update iphone 4s os 5.1 message "cannot connect server..." i've tried on air , i've tried thru itunes, , have never been able connect.

help, please?

 

a number of people here , on apple support forum have reported changing dns server on phone 8.8.8.8 , seemed allow it. guess can try.

 

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