You'll find this tab on the Home Screen. It's on the Apps page, though you may have to scroll down in order to find it. Doing so opens the Kindle Fire Store app. This option is in the Store menu. You may first have to tap on this page to find My Subscriptions. You'll find it in the list of current subscriptions. Tap the colored "Auto-renewal" switch to disable automatic subscription renewal for Pandora. Once you've completed the cancellation process, your Pandora subscription will no longer automatically renew. Your Pandora subscription will remain active until the end of the current billing cycle.

Summary:
Open the Apps page. Tap Store. Tap My Subscriptions. Tap Pandora. Turn off automatic renewal. Follow any additional on-screen instructions to confirm.