However, the "jailbreak" community that creates tools for making unsupported modifications to iOS usually devises a way to perform such a downgrade. Like Macs, though, an iPhone can only be downgraded to the version of software that it shipped with. For example: an original iPhone 3G may be able to be downgraded to iOS 2.x, and an original iPhone 3GS might be able to be downgraded to iOS 3.x. But an iPhone 4 will only be able to run iOS 4. It is also worth reiterating that these jailbreaking tools are not supported in any way by Apple. In fact, the company reserves the right to void your device's warranty if it discovers that you have used such tools, so tread carefully.

All things considered, it is rarely worth the effort to downgrade to a previous iOS version. If you are having troubles with a new version of iOS, your best bet is to seek official help from Apple, ask for community assistance in our Macworld forums, or simply try to make due until Apple inevitably releases a bug-fixing update.