A Draft and Approved purchase order can be cancelled or deleted before it is at the Received stage using the Delete or Cancel Purchase Order buttons on the PO detail page.
Cancelling the purchase order will leave the cancelled order in WorkGuru for future reference in the purchase order history. Deleting the purchase order will remove it completely.
Once a PO has been received, you can no longer delete it as that is the stage it has locked in the stock movement in the background. Reversing a received purchase order is done via a supplier credit note.