Just follow these instructions: 1. Under the heading "Cancel your subscription", click "View your options" 2. At the bottom of the following page, click "Click here to continue to the cancellation page" 3. On the following page, select one of the listed reasons for leaving 4. Then, click the button "Cancel Subscription"
Your subscription will expire at the end of the period that you have already paid for. After that, your Spotify account will return to the ad-supported service.