To update the credit card used to bill your account, you will want to locate the small profile icon to the left of George's picture. You can simply click on My Account to be taken to the account information area. Then click the Edit button next to "Billing Information".
You will need to log in before making any changes. If you're not already logged in, you'll be directed to the login page. After logging in, you will automatically be taken to the correct page.
Any updates made to your credit card will take effect immediately.