You can add new payment methods to your account from the Account page.
Follow these steps:
From the Account page, select "Payment Methods"
Tap "Add a card"
Fill out your card details and tap "Add" in the upper right
You can confirm that your card was added successfully by returning to the Payment Methods page and confirming that the new payment method is listed.
Note that payment methods are stored separately for each currency. If you change your account currency, you will need to update your payment method. Your payment methods are stored when switching back to a previously selected currency.