Cancelling your Source & Cast subscription will depend on where you initially subscribed.

For iPhone app subscribers, manage your subscription via your Apple Subscriptions.

For Android app subscribers, manage your subscription via your Google Play Subscriptions.

For website subscribers, manage your subscription via Stripe by going to the "Billing" menu item as follows:

If you cancel while on a trial, your trial will continue to be active until the expiration date, but it will not renew and you will not be charged.