If you're paying through Roku, you can manage your subscriptions here:
https://my.roku.com/account/subscriptions
You would have to cancel your current subscription and then resubscribe. Your current subscription would still be usable until the end of its subscription period. I don't believe there's any way to change from one subscription type to another.
If you're paying Britbox directly (recommended, if possible), you'll have to take it up with them.
