Hi 👋 Mike
You should be able to cancel the new card you ordered from the cards tab. Perhaps it's required to force quit the app and reopen it before this is possible. It's only possible to cancel the card before it's produced. If you don't see this option you may ask through support under Help > Support Center or on support@bunq.com.
You can replace your existing card (1 per year per person for free with Premium). Go to the card > Settings > Order a replacement.