To update your payment details will depend on the device you are using, iOS or Android. For further support visit the help centre:

If you are an iOS user, follow the Apple Support guidelines: https://support.apple.com/en-gb/HT201266
If you are an Android user, follow the Google Play Store guidelines: https://support.google.com/googleplay/answer/4646404?hl=en-GB&co=GENIE.Platform%3DAndroid