IPhone
Like your subscription, refunds are handled and decided exclusively by Apple via reportaproblem.apple.com. Please note that when you sign up for a free trial you receive an email receipt to indicate that you have started a free trial, but it does not mean you already have been charged.
Android
As iTunes, refunds are handled and decided exclusively by Google via your settings on Play Console. For that Go to Play Console > Menu 
> Account > Order History. On the order that you want to return, swipe left, tap on more and select "refund".