To access your billing, you'll need to head over to www.freemius.com and log in using the email and password you signed up with. Once on the page, click on the dropdown arrow of Log In and click on User Dashboard - Buyers.
Once logged in, select Renewals & Billing on the left
Select your product
A popup box will appear on the right where you can manage your billing account.
You can upgrade or downgrade your plan, cancel auto-renewal, update your payment method and find your license key if you've just installed your site.