Go to My account and click "Manage your credit cards".
Please note, you can also add a new credit card to your account during the payment process when upgrading or downgrading your plan.
Set a default credit card: Simply click on the cogwheel icon and select "Set as default" on the credit card of your choice.
Please note: if you only have one credit card on your account, it cannot be deleted. You will need to add a new one before you can delete the old one.