There's two ways to access Paramount+ - one is with their standalone app and one is through "The Roku Channel" app. You can add tiles to the Home Screen for both (search for Paramount under Streaming Store). Does either give you the option to "Manage subscription" when you highlight it on your Home Screen and press * on your remote?
Roku support, as it were, starts at https://support.roku.com/contactus. Choose the option for account/payments and there should be an option to call or chat, but the weekend may not be the best time to reach a human.
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