First of all, very sad to see you cancel your Pro plan... Did we do something wrong? Send us a quick note to email@example.com so that we can make ManyChat better.
Second, to unsubscribe them from the Pro version, go to Settings -> Billing and press "Cancel Subscription". This will make sure that you continue on the Pro plan until the end of your paid period but won't get charged for the next one.
Hope that helps!