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HI Cadebrewer, if you have a current subscription to Spotify you can manage it through your Subscription page. If you don't see an option to cancel then:
1) Did you ever enter any payment information when you signed up for your trial? If not, then don't worry. You can't be charged, no need to cancel.
2) Is it saying it will charge you $12.99? This means your subscription is through the App Store and you will need to cancel your Spotify subscription through there.
Hope this helps! Let me know if you need anything else.