you can do a restore. set it up as new, because if you have a 3gs, you cant jailbreak or downgrade. setting it up as new will make it so you have your warranty 100% back on there, because if you restore from a backup, your iphoen could contain crash logs, and then if a genius plugs it in, goes into xcode organizer, and looks at your phone's crash logs and sees a cydia crashlog, then your warranty is voided.