If you've associated a Purchase Order to a Client record or a Placement record and you need remove it, you'll first need to navigate to the proper record (Client or Placement).
Navigate to the Client record. Click Related and then Purchase Orders
From here you can see the existing Purchase Orders. Click in the PO number you are looking to remove.
From here you could delete the PO by clicking the Delete button. In this case we are going to look to remove its association to a Placement instead of deleting it. Click on the Placements tab.
Put a tick next to the Placement and then click Remove
The Purchase Order has now been removed from the Placement. Click Save and Close to continue.
You'll notice the Purchase Order still exists in the Client record and can be used again if necessary.