Look for the 'Billing' menu item on the site and click that.
You will be taken to a page where you will be invited to enter your email address to receive a secure link, so you can sign in and update your details.
Please use the same email address you signed up for the course with to avoid any problems.
We use a service called Thrive Cart, so do not be surprised if you receive an email link inviting you to sign in to ThriveCart.