I got the new iphone 5 and i restored my old iPhone 4?

So i got the new iphone 5 and i restored my old iPhone 4 to use as a regular ipad for music in my car. I restored it but now a message is coming up saying i can't activate it with out my sim card or whatever. So i called apple customer care and they said i could uninstall the update. So how how do i uninstall the iphone software using some kind of program or something.

You can uninstall an update.
you will need a active att sim to get past the screen, or jailbreak it if your on 6.1.2