iOS 5.1 won't install on iPhone 4

my iphone won't update ios 5.1
i've had year , can't remember ever updating it, didn't know had to
won't install new apps , stuff because have ios 4.2 something
when plug computer asks me if want update 5.1 , click yes.. , starts , restore , comes error message , stops..
there can make update?

 

what's error message?

more details more help.

 

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