I just upgraded my colleague iPhone 3GS from 4.0.2 to 4.0.1 because he need to jailbreak it with jailbreakme.com. The overall process is totally different from when I used to downgrade iPhone firmware a couple years ago. As a result, I think it’s a good thing to share this experience with the world today.
Requirements:
1. First thing we need to add
74.208.10.249 gs.apple.com
to the end of this file C:\Windows\System32\drivers\etc\hosts (Windows) or /etc/hosts (Mac OS X)
PS. you might need to run notepad with administrator privilege for Windows7
After this process, your iPhone firmware checking with Apple is optimized within local machine.
2. Extract firmware 4.0.1 with Winrar
3. Replace 8A306 with 8A400 in buildmanifest.plist and restore.plist
4. Open 4.0.2 firmware with Winrar (do not extract)
5. Copy all files in 4.0.1 firmware folder to 4.0.2 firmware in Winrar
6. delete all 001 dmg files.
7. Save this file to be used to restore .
8. Put the iPhone in DFU mode and restore with this new firmware.
9. wala, now you can jailbreak this firmware with jailbreakme.com
^__^