Login to your account, select the tab 'Account', then 'Payment methods', followed by 'Add a new card.' Once you've inserted your details, then select the last four digits of the card you would prefer to keep as your default payment method.
Login to your account, select the tab 'Account', then 'Payment methods', followed by 'Add a new card.' Once you've inserted your details, then select the last four digits of the card you would prefer to keep as your default payment method.