The current documentation on Roku.com states that there is a "manage subscription" action that can be found by selecting the channel and then selecting "*" on your device.
While this may be true for Roku managed subscriptions, it is not true for subscription that are not managed by Roku. Since Roku users deal with subscriptions directly managed by some provider AND with roku-managed subscriptions, it is important in any documentation to make this distinction.
I read this documentation and selected by mubi channel to manage it and there was no "manage subscription" action. It took me seconds to realize that this might not be a subscription managed by Roku. That turned out to be the case. Sure I figured it out, but it would be better to get the documentation right.
The documentation should be updated to distinguish between Roku-managed subscription and direct subscriptions.
https://my.roku.com/account/subscriptions
Don't see your subscription?
https://support.roku.com/en-gb/article/208756478
How do I manage or cancel a channel subscription?
The payment method linked to your Roku account allows you to purchase subscriptions to channels for popular services through the Roku Channel Store. An advantage of using your Roku account as the payment method for subscription billing is you can manage or cancel your subscriptions in one place – directly from your Roku® streaming player or Roku TV™, or by accessing your Roku account online. If you have a subscription that is not charged to your Roku account, please contact that service provider directly.
(emphasis mine)