How to wipe and reinstall your iPhone OS
Whether you've got a PC or a Mac, getting your old iPhone ready for resale means securely wiping all your data. This handy video shows you how to do just that.
Whether you're looking to sell your iPhone and you accidentally delete the operating system, or you want to downgrade from iOS 4, Device Firmware Update mode will help you out. Getting into it is simple yet tricky, so here's guidance on a fast way to wipe and restore your iPhone. Note that this requires iTunes (Windows | Mac), in case you're using an alternative program to sync your phone or iPod Touch.