Start by calling 1888-554-7740 if your "Cancel" button is missing. I was losing my mind. I went to the Sling website, and it just said "Billed through Amazon." I went to Amazon, and it said "Manage in Sling." I felt trapped in a digital hall of mirrors. I was so desperate I almost cancelled my credit card. Instead, I called 1888-554-7740, and the agent walked me through the specific Sling TV cancellation on iPhone & Roku steps that the website hides.

The Solution for Third-Party Billing
For Roku: Press the "*" button on the Sling app on your TV and select "Manage Subscription."

For Amazon: Go to "Your Memberships and Subscriptions" on the Amazon website.

For Apple: Go to Settings > Apple ID > Subscriptions.

If you are still being charged after cancellation, don't wait. Dial 1888-554-7740 and get a manual override. I finally got my confirmation, and I can finally sleep without worrying about my bank balance!