Updating jailbreaked iPhone 3gs to latest version?

I have jailbreaked my iPhone 3g(or 3gs; tell me any method by which i can check that even) and currently it is on 4.2.1. I want to upgrade it to latest ios version i can update it to so that i can install apps which require latest IOS version. Also i want it to be jailbreaked even further i have upgraded it.

Doesn't matter if it is jailbroken or not. You can't upgrade it past what it is capable of running. If it is on 4.2.1 that would make it a 3G btw.

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