You can change purchase order lines and create additional purchase order lines after you save a purchase order, but before you sync it with the host system.

To change a purchase order line, select the purchase order line and make changes, as needed. Choose the Save icon.
To add lines to a purchase order, choose the PO Line tab, and then choose the New icon. Information is displayed in some of the fields, based on the original purchase order. Enter information in the PO Lines pane, as needed, and then choose the Save icon.
To remove a purchase order line that was created before it is sent to the host system, select the purchase order line and choose Delete.

You can't change or delete a purchase order or purchase order lines after a purchase order has been synced with the host system.