To update a new expiration date on my existing credit card

You can view and edit your payment method by logging in here: My Account. 

• In the Update Payment Method section, you'll see your current payment method under, 'My Cards'
• Click the pencil icon to the right of the payment method (image below) that will make the payment method fields editable.
• You can enter a new expiration date, and will need to enter your CVV number (the 3 digits on the back) to confirm. 
• Click the box to the left: Apply this payment method to all active subscriptions.
• Click the green Save Changes button at bottom.

If you need to enter a new payment method, those steps are here.