Upgrading the iOS on my JB iOS 4.0 3GS

[cameronjpu]

So I need a little handholding here. I jailbroke my iPhone last year so I could take it overseas, and it worked great and I haven't had to use it much since I got home (replaced it with the iPhone 4). I've been using it a bit recently and have learned the hard way that my mail provider is really not happy about iPhones pre-iOS 4.1 using their exchange service.

So I need some help - I don't actually remember what jailbreak I used when I did it. I think it was the one that was entirely done through Safari, and I did back up whatever things are on the phone that make it easy to JB in the future through Cydia. I hope that makes sense.

So my question is what can I do to upgrade this iPhone a bit, doesn't have to be all the way, but as far as possible? I'm sure there is some info that I need to tell you - baseband or something? My modem firmware is listed as 05.13.04.

Please let me know if you can help guide me through this. I'm very technical, but I just don't know the terminology of the iPhone JB community, the Sn0ws and the rains etc. Thanks!

Edit: OK so no replies to that one. I am guessing that it's not possible to upgrade it to 4.01. That's all I need to do. I am unable to use my exchange email account on the device without that capability. Thanks!