If you want to update your billing address prior to placing an order, you can do that on your account dashboard. If you want to update your payment method (Paypal or credit card), you can only so that in the "billing" section of your cart. You can get there by adding /store/cart/billing to the end of the URL of the site you're purchasing on if you are in the middle of your purchase.
Here's an example from the Backstreet Boys website:
Here's a full list of steps if you've just started shopping:
1) Select the item you would like to purchase. Once the correct item is added to your cart, select "checkout."
2) If the next page goes straight to the "shipping" or "review" portion of the purchase page, click back to the "billing" section that is listed directly under the red line that says, "your cart is reserved for the next 20 minutes."
3) Once on the billing page you will be able to edit any existing billing/shipping/credit card information.
If you'd like to remove a saved card from your account, please let us know and we'll delete your saved payment information for you.