How to install:

-Open iTunes & connect your iPhone

-Click on the iPhone in iTunes

-Press shift on keyboard and click on restore or put iPhone on mode DFU and shift on keyboard + select file 3.1.3 Firmware.

Note: if the iPhone was already been jailbraked you don't need to put it on DFU mode. If the iPhone isn't jailbraked then you have to put him in DFU mode and restore it true iTunes.

If you don't know how to put it in DFU you can check on youtube how to do it.

-Select the 3.1.3 Firmware and install it

Note:

-Only works on iPhone 3G !

-Not on iPhone 2G, 3GS or iPod Touch !