From the change plan page you can see all active subscriptions for your account. You will want to select the red "X" on the far right hand side. After clicking on the red "X" you will be prompted to either "remove business & purge data" or "cancel subscription". If you do not see the red "X" you will need to take over billing as shown in this take over billing article.
Note: By selecting "remove business & purge data" you are NOT cancelling the plan but only removing the business from the subscription if you need to add another. However, after removing the business it will give the option to cancel the subscription.
To fully cancel the subscription you will want to select "Cancel Subscription & Purge Data" and that will change your plan to free and end your monthly payment.
Note: You will only want to complete this process after all campaigns have ended and are no longer active as well as after exporting all necessary information as it is purged on cancellation. Need help exporting? Click here.
If you're not sure when the current subscription will renew, you can always check your active subscriptions via your Account Settings > Businesses; or in the individual invoices for each payment under the Payments section of your Account Settings.