Go to Settings > Your Name > iCloud
Click Manage Storage > Backups for iOS 11 and iCloud Storage > Manage Storage for iOS 10.3
Select your device name
Delete Backup > Turn Off and Delete.
On your Mac:
Apple menu > System Preferences > Apple ID > iCloud
Select Manage and click on the Backups
With the backup selected, click Delete to remove the backup. Confirm that you would also like to turn off Backup if needed.