How to change primary email address to another email that already signup creative cloud express ?

Please go to Adobe Customer Service (make sure to sign in to your Adobe account first and allow popups on your browser):
https://helpx.adobe.com/contact.html?rghtup=autoOpen will automatically open the chat window.
Type "Agent" in the chat box to bypass the chatbot and be connected to a person.

You may also be able to contact Adobe by phone:
https://helpx.adobe.com/contact/phone.html