My Account -> Subscriptions -> View -> Change Payment
Enter your new payment details on the checkout page. Click the checkbox for Update the payment method for all of my subscriptions before saving if you have more than one subscription.
If your payment failed and you need to update your payment info:
My Account -> Subscriptions -> View -> Scroll down to the most recent renewal listed and select Pay. Enter your payment info on the checkout page and your renewal will take place. Update your payment info for other subscriptions that did not have a failed payment using the Change Payment method outlined above.