The billing portal holds your billing information, statements, and the details of your subscription. You can update your credit card, as well as download statements and even cancel in this portal.
A link to this portal was sent to you upon signup. The link does expire for security reasons; if it has expired, login to your account at muse-themes.com, click on your name, select "Your Account," and then scroll down to "Access Billing Portal."
If you haven't confirmed the subscription through the billing portal yet, you will receive an email asking you to "click to confirm" in order to enter. This does not mean that you will be charged again; it is simply required by the billing system we use.