canceling and/or downgrading subscriptions

Do customers have to go into paypal or their credit card company to cancel subscriptions or can they simply delete their listing to cancel a subscription?

Also, what if they want to downgrade a listing, how does that work? Say they’re featured and want to go to a basic plan. I’m assuming that automatically changes their paypal/credit card subscription payments/plan. So, if they go from a $20 a month plan to a $10 a month plan does all that update throughout? Or does it require them to do something via paypal?