You delete all GymYou data stored in your iCloud in the iPhone settings app. Be aware that iCloud does not only sync your GymYou data between devices, but also serves as a backup. You may loose all your GymYou data both on your iCloud and your iPhone by deleting it.

1. On your iPhone go to Settings > your name.
2. Tap ‘iCloud’.
3. Tap ‘Manage Account Storage’. 
4. Select ‘GymYou’
5. Tap ‘Delete data…’