While subcontractor POs and Bills can be found in the same locations as your other bills and purchases, a Subcontractor PO would have to be removed in the Plan&Track section of the job.
If you navigate to the job phase of your subcontractor, you will see a clickable link with the PO# and “Original Contract”. Clicking here will open a new window, showing the original contract amount, and a trash icon that will delete the subcontractor’s PO from this job.
This will revert you back to the stage of selecting your subcontractor, which will open a new PO and essentially start the process from scratch, as shown here.