If you’re adventurous and don’t mind the possibility of losing your current jailbreak, then feel free to head on over to the iPhone Dev Team’s blog and take a look at what you need to do. When updating to a new official firmware and then re-jailbreaking your phone there’s always the possibility that something could go wrong. You might lose your “jailbroken” status, and find yourself having to live with a non-jailbroken phone for a while. That being said, the iPhone dev team has issued a warning about this particular PwnageTool version.
The iPhone Dev team warns that if you do not have any battery issues with your iPhone 3GS, then you should not update to iPhone OS 3.1.3.
The team also states,
“iPhone 3GS users (regardless of unlock) should stay away from this and all 3.1.3 jailbreak tools unless you know you have your “SHSH hashes” backed up via Cydia.”
Since I’m not part of the Jailbreak community I am not 100% sure what the ‘SHSH hashes’ are, however I am positive that it has to do with the hashes for the previous versions in case something does go awry.
If you’re really willing to get started, head over to the dev team’s blog and read all the details and then get started.
Source: iPhone Dev Team.