Your billing and other account usage details (including your current plan, next billing date, billing history, as well as the number of users, brands and amount of storage in your account), can be access through the Billing & Usage page in your Sprout Studio account.
To access this page either go to Settings > Account & Branding > Billing & Usage, or more conveniently you can click on the "Billing" button in the notifications dropdown (attached to your profile photo in the header).
On this page you can also change and upgrade your plan as required to suit your business needs, update your credit card information, and other administrative tasks, including cancelling your subscription.