You can very easily update your credit / debit card information. Simply place your order as you usually do and enter your new card information instead of choosing the card saved on your account.
The new card will automatically replace the old one (which is deleted) in your account.
In order to change the key of your Custom Backing Track, you just need to click on the arrows next to the "Add to cart" button.
When you have selected your key, click on "Reload" and your new Custom Backing Track will be generated accordingly.
What happens if I've already purchased the tracks?
From your account, under "My Files", you just need to click "Change". The "classic" interface will show up and from there, the instructions are the same.