It's not possible to do this, they make out it is so you will jailbreak the phone. Those 3 mates of mine obviously tried rebooting to previous software etc, DIDNT happen. kept coming up errors on the phone little yellow box with black exclamation mark telling you that your software is newer than older version etc, totally ***** the phone up guys take my word for it. just cause you haven't experienced your problems yet, doesn't mean you won't
oh and another thing you might want to know about the iphones, is your battery will last a maximum of 2 years, and after then you have to send it back to Apple for them to replace the battery and if you do this and your iphone is jailbroken, you wont get it back and you will be blacklisted from using any other products from Apple.
My first 2 iphones my battery lasted 1 year (because of overcharging) and second iphone lasted (1 year 10months) charging the usual and using the tips on how to improve battery life lol. there good phones to use but they are ***** for battery, bluetooth etc