How upgrade iPhone 3gs iOS 4.2.1 (jailbreak redsnow)?

I have iOS 4.2.1 in my iPhone 3GS it's been jailbroken with redsnow and after restarting the phone I don't have to jailbreak it again.
I have to upgrade my iOS to any newer and better version.
To which version should I upgrade it, I can do a little bit with iPhone regarding jailbreaking so tell me some easiest way/version to upgrade it and also how, I mean which tools I need.

Update to 5.1.1 not ios 6.
goto jailbreakmatrix.com, click on charts, iphone, 5.1.1 firmware and download the redsn0w program it tell you, then run it.

Update to 5.1.1 and use redsnow.