How To UnJailbreak An IPhone 4s?

I bought my iPhone 4s from a guy who used it for a while and had it jailbroken. I'm not all that comfortable w it being jailbroken bc I don't want it to lock up on me one day so my question is, is it possible to unjailbreak it and if so, how do I do that?

You have to run recovery mode: http://support.apple.com/.../kb/HT1808

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