Need help on changing Yahoo! Id. (Crazy) Ex-girl friend won't quit emailing me.

Um, why not just get a new one.

Close that one and create a new one. I have had to do that many times. Yahoo won't let you change ID's, at least as far as I've researched.

It's a FREE service, so quit using your current one and sign up for a new one.
Or even better get a Hotmail of best of all Gmail instead.

Use the Block Addresses feature in Yahoo Mail to block her email.