HT4972: iOS 4: Updating your device to iOS 5 or laterLearn about iOS 4: Updating your device to iOS 5 or later
Currently Being ModeratedMay 23, 2013 7:24 PM (in response to hdvuq)
4.2.1 is the highest possible for the iPhone 3G.
Currently Being ModeratedMay 23, 2013 7:25 PM (in response to hdvuq)
An iPhone 3G can't be updated past 4.2.1.
Currently Being ModeratedMay 24, 2013 8:39 AM (in response to hdvuq)
Can't be done. Due to your device's internal CPU and RAM, it will always be limited to running apps that use iOS 4.2.1 or earlier. Be sure to keep backups of any vintage apps you use. Once a developer updates their app past your maximum iOS, a compatible version can never be downloaded again.