How to unjailbreak my iphone 4?

It is ios 5.1 and it is jailbroken. I want to unjailbreak it so that can update to ios 6.1

Restore it on iTunes.

Only way by restoring and by the way i do not recommend to upgrade to 6.1 for jailbreaking but if your upgrading just for use and not jailbreaken the you can upgrade but i don't recommend upgrading

Restore it, then jailbreak it again

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