You can update the payment method on your subscriptions if you paid by credit card. This does not work for recurring payments set up in Paypal. To update your payment details login to the customer portal where you access your purchases. Find the product you want to update and click View Details. Scroll down to the Subscription Information section and click the orange Update Billing Info button as shown here:
You will then be taken to a page to update your information. Please note that this may not work with all payment processors. If it does not work, you will have to contact the seller as they may be able to update your information directly within their payment processor account.