The Subscription tab is available from the Profile screen. You can click on the Subscription tab at any time to see your current subscription level, your payment details, the number of licenses on your account, your access to the API and Green Grading, as well as the payment history on your account.
You can select the "Change Plan" button to pick another plan, or cancel your current subscription.
You can select "Change Card" to enter new payment information. It will open a window that will ask for your email address, card number, expiration date (MM/YY), and CVV code. The CVV code is the 3 digit number on the back of your card.
Please see here for pricing guide of all three plans available at Tastify (link).