How do I switch to a lower-priced subscription after being charged more?

Today I got charged for a subscription for a year that was offered to me a week ago with a free trial. It was on an app I already had for it to become premium. I planned to cancel and thought I could up until the day being charged, but it was the day before. Today I got a promotion when logging into that app for same premium version that is 40$ less. How do I change to that? 1 free week was not worth $40 more. If I had both options I would not have chose the 7 day free trial.