Forgot unlock passcode. How do you reset?

just restore by itunes but beaware u must remember your icloud id and password the iphone first created

If you have forgotten the passcode for the lock screen, follow this article:

iOS: Forgotten passcode or device disabled after entering wrong passcode

If you have problems with the Activation Lock feature in iOS7, start with this article:

iCloud: Troubleshooting Find My iPhone, iPad, iPod touch, or Mac