If you're the account owner, click on your avatar in the upper right corner and then click the “Account & Billing” link in the drop down menu. Here, you'll see a short snapshot of your account, the available plans, and previous invoices too. If you click the link “Billing”, the drop down menu gives you an option to click “Billing Information”. When you click “Billing Information”, you can update the card on file and save that for future billing cycles.
If you’re the account owner, you can go to this URL to update your card: