iOS4 though also is a bit of a battery drain and can make you run for the charger faster than iPhone OS 3.1.3, however, with the slew of new features it is definitely a exciting upgrade. So how do you upgrade to iOS4? Read ahead.
How To Update iOS4 on iPhone 3GS, iPhone 3G and iPod Touch
iOS4 will be released to the general public on June 21, this is different from the iOS4 GM release which was released for developers.
To upgrade to iOS4 on your iPhone 3GS, iPhone 3G and iPod Touch, you will first of all have to upgrade iTunes 9.2, you can download or upgrade iTunes 9.2 using the Apple Update manager.
Once you have updated your iTunes, connect your device, accept the terms and visit the summary page for the device.
On that page, you will see a button to "Check for Update", click on it to see if the new iOS4 update is available for your device. If there is one you can then follow the normal upgrade process and your iPhone 3GS, 3G or iPod Touch will be updated to use iOS4. Enjoy.