a) Launch the Google Play Store app on your Android phone.
b) Click your avatar - Subscriptions - tap Castbox to cancel it.

Here is the link from Google Play about how to cancel, for more details, please check: https://support.google.com/googleplay/answer/7018481?hl=en&co=GENIE.Platform%3DAndroid

2. Apple App Store

Please go to your App Store app on your iPhone - "Today" tab – click the figure button in the top right corner of that page - your name and ID box - Subscriptions - find Castbox and cancel it.