Once you've logged in successfully, select the Content tab
Click on Purchases on the top or scroll down until you see the list of purchases
Click on the purchase you want to manage
Click on Manage payment on the right side of the screen under the product picture
You will be redirected to the Payment management screen. Should you have a past-due payment you will be shown what is due and can pay the balance. Otherwise, you are free to update the payment information for when the next payment is due.