If you signed up on your iPhone:
Go to Settings > Apple ID, iCloud, iTunes & App Store > iTunes & App Store.
Tap your Apple ID at the top of the screen.
Tap View Apple ID. You might need to authenticate your Apple ID.
Tap Manage Payments.
Then make your changes.
If you signed up on your Android device:
Open the Google Play Store.
Tap Menu > Account > Payment Methods.
Find the subscription you want to update.
Tap Update.
Then make your changes.