The new iOS 4 for the iPhone 4, 3G, 3GS and iPod Touch is out now and is available for the normal users. You could now upgrade iPhone 3G/3GS to iOS 4 using iTunes. If you have doubts, then here is how to update iPhone 3G/3GS to iOS 4.
1. Download iTunes 9.2 from here.
2. After installing iTunes, connect your iPhone 3G/3GS to the PC and select it in iTunes.
3. Now under Summary tab, click on ” Check for Update “.
4. Now select Download and Install to start upgrading your iPhone 3G or 3GS to iOS 4.
5. Wait till iTunes finishes iOS 4 update.