You can delete your BetterYou account directly from the app.
Go to Profile
Tap on Account
Tap on Delete Account (below the Sign Out button)
Follow the prompts
Once you've fully completed the steps in-app you can fully uninstall the app.
Canceling/deleting your account will permanently delete your data after 48 hours
If you change your mind during that time and re-sign into your account your data will still be available.