![How to: Upgrade iPhone 3GS 4.2.1 (BB 05.13.04) Using Custom Firmware [Guide]](https://lh3.googleusercontent.com/blogger_img_proxy/AEn0k_vUAsMkAx8GwVabWAEuei52K4LGv-4PtFkLcQU-gZ5gnkqhOOkzcrAm-ho433BvSWIe-FtBxCDLKUY3-6R6i9MiMaAol37BmNMzz3kEwfFlLC0fgkpM267O=s0-d)
by this you don't Have to worry about future upgrade. Because once you upgrade your baseband to 6.15.00 you cannot update your phone with apple original firmware in future.
Before you Start this procedure. Make sure you have BB 05.13.04.....or below...
See the image...
Before you Start this procedure. Make sure you have BB 05.13.04.....or below...
See the image...
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