Highlight the app's icon on your Home Screen and press * on your remote and there should be an option to "Manage subscription" if the subscription is through Roku. You can also log into your Roku account at https://my.roku.com using the email address that's displayed on your Roku device under Settings->System->About and you can manage the subscriptions you have through Roku there.
Note that subscriptions are generally prepaid an non-refundable, so don't expect any prorated refund.
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