How do you change your default payment method in the app store for subscripions?

On the Mac, open the iTunes/Music app. In the iTunes Store click Account in the Quick Links on the right side of the store and log in. In your account details edit your payment method.

M1 Mac mini, macOS 12.0.1; Watch, series 5 GPS/cellular, watchOS 8.1; iPad Pro 2nd Gen, iPadOS 15.1; iPad Air 2, iPadOS 15.1; iPhone 12 Pro, iOS 15.1; TV 4th Gen, tvOS 15.1; HomePods, audioOS 15.1 and Airport Express w/ Airplay 2