So you want to use iOS4 a little bit early. You aren't a developer, and you want to use it on your iPhone. The good news is that it's pretty easy, but you need a few things: A Mac, iTunes 9.2 Beta(released for developers, and not yet available for Windows) and a compatible iPhone or iPod Touch. If you have all of that, you are ready to roll. Here are the links for downloads:
iTunes 9.2 Beta
iOS 4 GM seed ‚Äì iPhone 3GS, 377.81MB
iOS 4 GM seed ‚Äì iPhone 3G, 292.3MB
iOS 4 GM seed ‚Äì iPod Touch 3G, 356.53MB
iOS 4 GM seed ‚Äì iPod Touch 2G, 308.74MB
Warning: Keep in mind that this isn't a final release, this is made for developers who want to test their product on the hardware. We have also read that some people's photos didn't come back, even with a restore. You may try emailing them to yourself or finding a way to make sure they stay on your computer while you experiment with your device. As always, back up your files and save ANYTHING of value. Finally, only try at your own risk, if you have any doubts, keep in mind that iOS 4's public release is less than a month away.
1) First, you must go to the following site to download iTunes 9.2 Beta and the iOS4 firmware. Make sure that you download the firmware package that matches your device (i.e. 3G, 3Gs, etc.)
Download iTunes 9.2
2) After you have downloaded both software packages (and it will take a while), you must install iTunes on your computer.
Install and run iTunes 9.2:
3) Plug in your device, but cancel the sync.
4) Choose the device that you want to install the new operating system on (In our case this was an iPhone)
5) Hold down Alt, then click on the Restore button.
6) This gives you the option to choose the software you want to restore the iPhone from, choose the iPhone 4.0 .ipsw file that you just downloaded.
7) Once you do that, you will be waiting for several minutes as the new firmware is installed on your device. This will take a while, so make things a bit easier and make yourself a quick sandwich and grab a soda.
8) After the software installs you will notice that your iPhone now has iOS4, but your Apps are missing. Ruh roh!
9) No worries. Now you have to do a restore from backup. This will also take a while. Watch a bit of TV or finish up your soda while you wait. This will give you back your Apps, and hopefully your pictures. If not, we hope you followed our advice in the warning above.
10) Once everything is installed, iOS4 is in your hands. Enjoy the new features, but keep in mind that compatibility can't be guaranteed with every App.
Follow this guide to save your iPhone from bad installation/restore.