It depends on whether the subscription is through Roku or directly with a provider. If it's through Roku, you should be able to highlight the app on your Home Screen, press * on your remote, and choose the "Manage subscription" option. You can also log into your Roku account at https://my.roku.com using the email address displayed on your Roku device under Settings->System->About and manage any subscriptions you have through Roku there. If the subscription is directly with a provider, you'd manage the subscription on their website.
How to turn off auto-renew and cancel a subscription | Official Roku Support
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