To delete an entire Computer Backup, open myCloud on the web and click on "Files". There select "Computer Backup". You will now be redirected to the overview of all backed up computers.
Here you select the three points for the backup that you no longer need. You can delete it by selecting "Delete backup".
In order to complete the deletion process, enter the exact computer name and confirm the deletion with "Delete permanently". Your backup including the archive will now be deleted.
This deletion cannot be undone and the data cannot be restored afterwards.