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
