With the new iPhone 3.1 update, iPhone 3GS users can no longer downgrade from iPhone 3GS 3.1 firmware to iPhone 3GS 3.0. Apple wants users to stay with the most current firmware. There is no way to jailbreak iPhone 3GS 3.1. No custom firmwares are able to be added on iPhone 3GS either. Fortunately, Saurik, the creator of the Cydia application has come up with a solution. It only works for the few that were able to use Cydia's "on file" system and "hash" their ECID (ECID is the unique chip ID that each iPhone 3GS device has). If you were one of the few, read on below to find out how to downgrade iPhone 3GS 3.1 to iPhone 3GS 3.0. If you were not one of the few, then you are still lucky. iPhone 3.1 can still be jailbroken, but you'll need to wait until a full jailbreak solution is out. I'll post the guide here once it's out.
How to Downgrade iPhone 3GS 3.1 to iPhone 3GS 3.0
If you are on Windows Vista or Windows 7 you will need to run Notepad as Administrator and then locate the hosts file and edit it. Then move on to step 4.
1. Press Windows + R on your keyboard or go to Run.
2. Type in the following: C:\Windows\System32\drivers\etc\hosts.
3. Click OK. You should now get an Open With window. Click on Notepad and then click OK.
4. Your hosts file will now open in Notepad. Make a new line below 127.0.0.1 localhost and copy and paste in the following:
5. Click File - > Save in Notepad. Where it says "Save as type" make sure you select All Files and then click Save.
6. Download iPhone 3GS 3.0 (iPhone2,1_3.0_7A341_Restore.ipsw) and save it to your desktop. If you haven't already, download iTunes 9 and install it.
7. You must now put your iPhone 3GS in Recovery mode. Connect your iPhone 3GS to your PC. Hold down the top iPhone 3GS button and the bottom home button for 10 seconds, let go of the top button but continue holding the bottom home button until iTunes displays the following message: "iTunes has detected an iPhone in recovery mode. You must restore this iPhone 3GS before it can be used with iTunes." Your iPhone 3GS screen should be black.
8. Open iTunes, press shift on your keyboard and click "Restore". Locate the iPhone 3GS firmware 3.0 file you downloaded in step 6 and iTunes should now begin restoring it.
- If you get unknown error (3002), it means your Cydia on file system was not hashed. You cannot ever downgrade to firmware 3.0. Just stay on iPhone 3GS 3.1 firmware for now and wait for the iPhone 3.1 jailbreak to be released.