First make sure the card you want to use is fully up-to-date (expiration, etc.) in your PayPal account's funding sources. Then see if you can edit the existing subscription to select that as a funding method. If not, you can try canceling and restarting your subscription as explained here (same procedure, you'll just be using the same PayPal account for the "new" subscription):
http://www.e-junkie.com/ej/tips.subscription.paypal-account.htm
PayPal will always try to fund subscription payments from your PayPal account balance first, then they will turn to your subscription's preferred funding method to cover any unfunded remainder of the payment.
