If you subscribed via Google Play
You can modify your payment method by following this link: Manage Payment Methods
Or by following these steps on your phone.
Open the Play Store app
Tap the profile icon at the top right of the screen
Tap Payments & subscriptions
Tap Payment methods
Make the changes you want.
If you subscribed via Apple App Store
You can modify your payment method by following this link: Manage Payment Methods
Or by following these steps on your phone.
Open the Settings app
Tap your name
Tap Payment & Shipping (You might be asked to sign in with your Apple ID)
To add a payment method, tap Add Payment Method
To update a payment method, tap it, then edit the information
To remove a payment method, tap it, then tap Remove Payment Method
To change the order of your payment methods, tap Edit and drag the icon to change the order - The first payment method will be the default