How can I reset a passcode locked iPhone?

Episode 1172 (18:09)

Paul from West Covina, CA
Apple iPhone 4

Paul recently bought an iPhone and the passcode has suddenly stopped working. Leo says that a full restore can solve the issue. He can connect it to a computer and then go into iTunes. But he may need to go into something called "DFU mode." This is a more serious reset of the phone. Paul should search YouTube for DFU and see how it's done.