You can cancel any time, right in the app.
Open Settings.
Go to the Billing tab.
Hit cancel and follow the prompt.
A couple of things worth knowing:
No refunds. Cancelling stops your next billing cycle — it doesn't refund the one you're already in. You keep full access until that paid period runs out.
Your downloads are yours. Every track you pulled stays on your drive after you cancel. Downloads are yours to keep forever.
Changed your mind? You can resubscribe any time from the same Billing tab.