how to manage icloud data

Settings > Click on your name > iCloud > Choose what apps you don’t want to sync with iCloud and also click on iCloud Backup to see what apps are backed up. You could turn the toggle off of any specific application is consuming more storage and remove its data from iCloud

It sounds like you might be asking how to downgrade your plan. If so, start here:

Downgrade or cancel your iCloud+ plan - Apple Support

If that's not what you mean, please explain in more detail exactly what you're trying to do and what happens when you try.

When you set up iCloud, you get 5 GB of storage automatically. You can use that space for your iCloud backups, to keep your photos and ...

Manage your iCloud storage