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How to downgrade iPhone 3G(S) from version 4.x.x to version 3.x.x.

Why do I want to do that? you may asked.
 1. If you don't know why, then why bother.
2. Because version 4 is way way to bloated for ya old iPhone. If ya aint gonna miss the extra feature, or if you don't know what's the different between 4 and 3, then why bother.

Article from Life Hacker