Hello @Liza48,
If you have a confirmed reservation for December 11 to 18 then you've already paid in full. When you make a reservation the credit card you have as the payment method is charged immediately.
If you add a new card now an existing reservation can't be charged to it as you've already paid with the first card.
If you still have issues you should contact Airbnb customer support.
Log in and click on "Help" and then type "contact airbnb" in the "Ask a question ..." box. Then click on the "I want to contact Airbnb" link.
Then click on the "Contact page" link near the end of the page.
You can also type "contact airbnb" in the "Search the community" box on these forum pages where you posted your question. Scroll down a little in the list of suggestions that appears and click on the "Contact Airbnb: A Community Help Guide[UPDATED]" one. That lists phone numbers and ways to contact them via Twitter or Facebook.
Steve.