If you want to delete an existing iOS backup from your computer for some reason, it is really easy to do by following these simple steps::
iOS backups created by iTunes generally are in the default locations,
/Users/yourusername/Library/Application Support/MobleSync/Backup on macOS
C:\Users\yourusername\AppData\Roaming\Apple Computer\MobileSync\Backup on windows
iBackup Viewer will list all available backups under the default backup folder, you can reveal the location of a backup by click the "folder" icon
After revealing the location in Finder or Explorer, you'll see folder and files hold the backup data. Go to upper folder and delete it. The data will be delete completely and not recoverable in general way.