To unlock your iPhone 3GS, you need to have Cydia installed in order to access UltrasnOw. If you don’t have Cydia installed, install RedsnOw and walk through the process. You can then unlock your iPhone by following these steps:
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Open the Cydia app.
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Press the manage tab at the bottom of the Cydia screen.
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Click on Sources.
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Press the Edit button on the top right hand corner of the screen.
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Press Add on the top left hand corner of the screen.
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When the text box appears, enter Cydia/APT URL.
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Add http://repo666.ultrasn0w.com/ as the URL.
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Press Add Source.
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Press Return to Cydia.
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Press Done in the top right hand corner of the screen.
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Select repo666.ultrasn0w.com under “Entered by User”.
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Press select UltrasnOw under the repo.666.ultrasn0w.com.
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Press install in the upper right hand corner of the screen.
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Press Confirm
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Choose “Reboot Device Button” when it comes available.
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Your iPhone 3GS is now unlocked.
Resource: iPhoneHacks