To update your credit card for your plug&paid Supercharged plan visit your "Settings" page and click the "Subscription Plan" tab. Click the "Update Credit Card" button shown under the "Plan Settings" section to initiate the process with Stripe.
In the Stripe modal window that will pop-up fill-in your new credit card information and click "Update Card". The process is handled entirely by Stripe. plug&paid will not view or store your credit card information.