Once a Purchase Order is canceled or closed, it cannot be reopened. Therefore, there is no way to edit the order as it must be in an "Open" state.
Typically, a Purchase Order is closed when the items that are still open on the PO are no longer expected to be received. We generally recommend that you leave the PO Open if you still expect to receive items against it and close it when the items have been canceled.
A PO can be canceled only if it has no completed receipts against it.