iCloud & iCloud Photos · iCloud storage space and plans
How do I delete an app's data from iCloud to free space?
Short answer: In System Settings → your name → iCloud → Manage, select the app and choose Delete Data. It removes that app's data from iCloud and every device that syncs it, permanently — so only do it for apps you no longer need.
Open System Settings → your name → iCloud → Manage, select the app in the list and choose Delete Data (or Delete Documents and Data). This removes that app's data from iCloud and from all your devices that sync it — it can't be undone, so only do it for apps you no longer use or whose data you've saved elsewhere.
To stop an app storing more without deleting what's there, turn off its iCloud syncing instead — see stopping an app using iCloud.