iCloud storage

You can download synced files, contacts, notes and photos from iCloud and delete them from there whenever you want.

You cannot modify a backup once it's been made however, but you can change what is backed up the next time from the iPhone Settings by turning on or off the features and apps to back up in Settings ➜ Account (top of settings) ➜ iCloud ➜ Manage Account Storage ➜ Backups ➜ Device Name.

From there let the list of apps and features load, and you can turn on or off the features and apps you don't want to back up.

Macs don't backup to iCloud at all, so nothing to turn off.

If you have option of Syncing Desktop and Documents turned on, you can turn it off on the Mac on System Preferences / Settings ➜ Apple ID ➜ iCloud Drive ➜ Options ➜ Desktop & Documents and make sure its unchecked.

Can I also turn off backing up my Mac in this settings pane, or do I need to go somewhere else?