The Eco dyno plan is a flat fee for a fixed amount of dyno hours per month. If you cancel the Eco plan during the month, you retain the pool of hours until the end of the month. You can't get a partial refund on unused hours you have already paid for.
After canceling your plan, at the beginning of the next month, any Eco dynos you have will be automatically scaled to zero and remain that way until you re-subscribe to the Eco plan or choose a different dyno tier.
See our blog post and the Low-Cost Plans FAQ for more info related to our new plans.