Apple iOS 4.0 final release has been available for iPhone 4, 3Gs, 3G, and iPod Touch.So if you owned iDevices you can easy upgrade your iPhone OS to iOS 4.0 either via iTunes or by manually download the .ipsw file of iOS 4.0.
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Here is the manual steps to downgrade iOS 4.0 to 3.1.3. Note before starting to downgrade iOS 4: you will need the ECID SHSH certificate for firmware 3.1.3.
Steps to Downgrade iOS 4.0 firmware to 3.1.3
1- Windows users: Navigate to c:\windows\system32\drivers\etc\ then double click on the “hosts” file and open in a notepad. Mac users: Navigate to the path “/etc/” and open the hosts file with TextEdit. (I’am in Windows and using NotePad2)
2- At the end of the “Hosts” file simply add the string “188.8.131.52 gs.apple.com”.
3- Then set up your device in recovery mode by simply turn off and unplug your device from your computer >> press and hold the Home button while reconnecting the USB cable to your iPhone >> after iPhone is connected it should power on the device >> continue holding the Home button, and you will see the iTunes logo appear and then finally a “Connect to iTunes” will be displayed >> release the Home button now.
4- Now open iTunes and do a normal reset by holding down the SHIFT key (Windows) or Alt key (Mac) and clicking restore in iTunes >> now select the original 3.1.3 firmware and wait for the completion
5- So now you’ve downgraded your iPhone / iPod Touch firmware back to 3.1.3
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