If you want to add a new credit card or edit your existing card then head over to Settings > Billing & Usage to see your billing preferences.
How to Edit Your Credit Card Information
Next to the credit card, click on the 3 dots to edit your current credit card.
From here you'll only be able to edit the name, expiration date, and zip code.
If you need to add a new Payment Method, then follow this tutorial.
How to Add a New Credit Card
Click on the Add Payment Method button to open a new screen. From here you'll enter in your new credit card.
After you've typed in your new card, then click Add Payment Method.
How to Change Your Primary Credit Card