Your subscription for the Courtney Black App is processed through your App Store, so to change details you will need to follow the steps below dependant on your device type and provider.
If you have an Apple Phone:
https://support.apple.com/en-gb/HT201266
If you have an Android / Google Play Phone:
https://support.google.com/googleplay/answer/4646404?co=GENIE.Platform%3DAndroid&hl=en