https://support.apple.com/en-us/HT201266

on your iPhone, iPad, or iPod touch

1. Open the Settings app. 
2. Tap your name, then tap iTunes & App Store.*
3. Tap your Apple ID (which is usually your email address), then tap View Apple ID. You might be asked to sign in.
4. Tap Manage Payments. (If you’re using an older version of iOS, tap Payment Information.) Then add, update, reorder, or remove your payment methods: