Firstly, you just need to to login, enter your email address and then click "Continue" Enter the password associated with your account and then click "Continue"
From the dashboard on the left hand side, you'll see some text that will display the last four digits of your card that's currently stored.
If you'd like to remove these card details just below is a link that says "Update your payment details" Click on this, which then gives you the option to remove this card and enter new details. Click "Remove this card"
If you then look on the dashboard in the same place, you'll see that there are no stored card details listed.
It will still give you the option to update your payment details. So prior to a renewal for example, you could add in your card details and then go back through and repeat this process to remove them again if you wish.