You must cancel your subscription through a web browser, as you often cannot do this inside the mobile app.
Step 1: Log in to your account at spotify.com/account.
Step 2: Scroll down to the Your Plan section.
Step 3: Click Change Plan.
Step 4: Scroll to Cancel Spotify Premium and click Cancel Premium.
Result: You will keep your Premium features until your next billing date, after which your account switches to free.