If you're subscribed through Roku, you can't uninstall it from the device until the subscription is cancelled and the current period expires. You can uninstall it using the mobile app. Search for it within the mobile app and if you select it from the search results it should give you an option to uninstall it. Keep in mind that uninstalling an app doesn't cancel the subscription and you must restart your Roku device before you reinstall it or you won't accomplish anything. For a Roku TV, perform a System restart found under Settings->System->Power before you reinstall it.
Roku Mobile App - Free for iOS® or Android™ | Roku