You can reinstall the app and delete all cache from your iPhone device and your iCloud. However, please note that it will erase all data, and you will be able to start fresh. We will not be able to restore it.
1. Go to your Settings and scroll down to iCloud, tap on it:
2. From here, tap on Storage:
3. Choose the Fasting Tracker from the list:
4. Tap on Delete Data from iCloud.
Please don't forget to reinstall the app as well.