To manage your cards, go to Your account (profile picture) on the side menu and click on Billing.
Under Your cards, you will see a list of the cards added to your account. If you only have one card added, it will be your default payment method. Now if there is more than one, you'll be able to set one of them as your default payment method by clicking on the Default button on the right side.
To add a new card, click on Add card, fill in your card details, and click on Add.
Keep in mind that you can't edit your card details, so should you need to modify them make sure you delete the card and add it again. To delete a card simply click on the Delete button next to it.
If you have an active Premium subscription, you'll need to have a default payment method. If you have only one card added, you won't be able to delete it unless you add a new card or cancel the subscription.