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I have tried numerous times since the update was available and it always says "Unable to Install Update. An error occured installing iOS 5.1.1" I am connected to the internet, so do not understand the problem.
I have also tried downloading the update from my laptop but it will not let me do so through that either.
I had this problem with an iOS update once. It would download most of the way (thus eating up my quota), then bug out before the end.
Suggestions at the time ranged from trying another day (in case the update servers were overloaded) to asking my nearest Apple Store to download and install the update for me.
My nearest Apple Store was 250km away, but yours might be closer. Or the download might work better on another day.
Good luck. :)

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