1. Backup your data using iTunes.
2. Download PwnageTool 4.01 [mirror]
3. Download iPhone 3GS iOS 4 Final Firmware
4. Launch PwnageTool 4.01 and select your device
5. PwnageTool 4.0 will automatically detect the correct iOS 4 [ipsw] for your iPhone 3GS or 3G or you can use the browser to locate it.
6. When PwnageTool asks “Do you have an iPhone contract that would activate normally through iTunes?”, click on “No” if you are not on official carrier or “Yes” if you are on official carrier.
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