Upgrade Spirit 3.1.3 to IOS4

Hi

My girlfriends company gave her a Berry phone, and she gave me her Iphone. And I want to upgrade it to IOS 4. This is the actual phone status:

Iphone 3GS 16GB All ready activated with ATT
Jailbreaked with Spirit (she did it herself) SHSH saved on Cidya
Version 3.1.3 (7E18)
Boot version 359.3
Firmware 05.12.01

I have been reading for the past days about the all jailbreak options. She never upgraded her phone and at this time I was reading about staying away from the last upgrade. Many of the websites have tutorials that recommend to restore and then install, but none of them mention if you used Spirit for Jailbreaking the phone.

I want to make sure about the correct steps to do it if it can be done. I read about using custom IPSW but my concern is that can I use one of this IPSW file to upgrade to IOS4, because all of them says Restore.

Please don’t flame me because of my question telling me to Google it :(, english is not my first language so I triying to explain the best I can.

All help will be appreciated.:D

Pwnagetool custom ipsw does not support "upgrade" only "restore". It will likely never support "upgrade" because upgrading breaks Cydia and programs installed via Cydia.

The normal pwnagetool process is to:

1. sync your phone with iTunes, make sure that iTunes backed up the contents of your phone.
2. make a copy of the backup for safety's sake.
3. make sure your phone is (if necessary re-run pwnagetool and make a new ipsw, then answer no when asked if your iPhone is pwned.
4. Confirm it is definitely old bootrom.
5. Option-click restore and select the custom ipsw you made earlier
6. when restore is complete, either choose "restore from iTunes backup" (this is effectively upgrading) or "set up as new iPhone" (this is recommended if you experience poor battery life or instability)