How can I change my credit card info for my PayPal subscription for the WunderCounter?

Here's what you need to do. Go to your main stats page and click on "Upgrade to Premium/make a payment" (which is under the "options" menu). Click on the button for the payment plan that you'd like to sign up for.

Create a new subscription based on the new credit card. When you're done you may either cancel the old subscription or let me know and I will cancel it on your behalf.