Welcome to the forum! Good news and bad news....
You can't change the factory code only add new ones. If you are lucky, the plastic card that had the code on it is in the owner's manual. If your luck is like mine, it's long gone. In which case you have two options:
https://www.youtube.com/watch?v=AAKbq_4eSNI
or take it to a Ford dealer and have them hook it up to the computer to look it up.
Once you get the factory programmed code, you can add your own.
