Here’s the Step by step Guide
1. Open System Preferences from the dock, your Finder
2. Click iCloud and sign in if prompted.
3. Click Manage… in the bottom right corner of the window.
4. Click Backups in the menu on the left.
5. Click the backup you want to delete.
6. Click Delete on the bottom left the pane.
7. Click Delete in the pop-up.
8. Click Done or repeat steps 5 to 7 to delete another backup file.