You can update your payment details at any time as follows.
From there, you will see a Green link button under Payment Details called 'Edit Account'.
Clicking that link takes you to your hosted payment pages where you can modify / update your account details. You may be able to go there directly at http://stockopedia.recurly.com/account . Please note that you may need to request a password reminder to access this site as it's hosted elsewhere for security reasons.
If you have any trouble, just raise a ticket with us via the Green Help & Support button on the site, and we'll help you out.