The default Ship To address on purchase orders is printed from the address in the Location assigned in the purchase order header as highlighted below.  

The address can be entered or changed in the System Setup - Inventory - Locations screen.  If you enter or change the address on the location, it will change the ship to address on the printed purchase order.  If you have multiple manufacturing locations or warehouses, you can select the location on the PO and it will print the address associated with that location.

If you want to ship directly to a vendor, customer or one time shipping location, you can use the Drop Ship feature on the purchase order to select a different ship to address.