Hello! My credit card was hacked, cancelled and replaced with a new card and number. The old credit card had 2 partially paid reservations pending. I managed to contact an Airbnb support personnel online and she managed to change the credit card number attached to the 2 reservations for me.
To reach that page, click 'Help' on the top right corner, scroll down to bottom, click 'Visit the Help Centre'. Scroll to bottom of Airbnb Help Page, there will be a question 'still need help?', click the green box next to it that says 'Contact Us'. Choose which trip that you are having problems with, and use the drop down box that says 'My host wasn't able to answer my question'. When asked what problem you have, choose 'reservation', then 'credit card', say that you have already contacted your bank, type in your question, and wait for a pop-up box with a support personnel to reply to your query. It was quick and painless. Hope this helps!