To update your payment method simply click here, or navigate to "Manage Payments" from within your account settings.
If you originally signed up with PayPal, you won't be able to change the payment information directly on that page. If you want to continue to use PayPal and just need to update your card on file with them, go to Paypal.com and your payment will automatically update.
If you'd like to completely switch from PayPal to a credit or debit card, you will need to cancel your membership and sign up again. Please contact customer support for assistance if needed.