You can perform the below steps to delete backup of any container through Parallels Virtual Automation (PVA):
1) Go to Infrastructure , then Click on the Hardware node
2) In the right pane, click on Backups tab
3) Click on the virtual environment whose backup you want to delete.
4) On this screen you can see a list of available backups for the Container, select the listed backup and click on Remove Backups
For more information - refer to the documentation