my i4 is jailbroke, can i up date to 4.0.2 ? will it hurt my phone?

can i update my iphone for to 4.0.2 ? will it hurt my phone? how do I "unjailbreak" my phone? thanks

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I have a iphone 4 that's jail-broken, Why is it when i download a battery app from cydia then select it from winter board it doesn't work ? Am I doing something wrong?