You can change or add a payment method by following these instructions:
On the Home screen, tap 'Settings' ⚙️
Tap iTunes & App Store.
Tap your Apple ID. (You might need to sign in with your Apple ID.)
Tap View Apple ID.
Tap Payment Information.
Change your information and tap Done.
On your Android device, open the Google Play Store
Tap ☰ Menu → Subscriptions.
Find your Straightaway subscription:
For active subscriptions, tap Manage
To fix an issue with your payment method, tap Update
Follow the on-screen instructions to update your payment method.