Your payment information is stored at Stripe.com. Stripe shows Boom Learning your type of card (Visa/Mastercard/etc), the last 4 digits of the card, and the expiration date. Stripe's security is audited and it certified to PCI Service Provider Level 1, the most stringent level of certification available in the payments industry. (Learn more directly from Stripe.)
To update or change your card number go to My Settings (top right menu). The screen you are looking for look like this:
PS: If you are trying to buy an upgrade by credit card you will need to send us an email for manual entry.