Use the Bill To Address tab (Administration > Inventory > Vendor Purchase Orders > Select a search option > Open a purchase order) to view and update the default Bill To Address on the purchase order.  By default, the accounting Default Bill To address is used on the purchase order; however, you can update the address manually or by selecting from a list of addresses.  

Click Update to edit or change the bill to address on the purchase order.  You can manually enter a new address or select an address from the Address Options drop down. This menu includes the Default Bill To address as well as all stock site addresses.   Note:  Manually updating the address does not permanently change the stock site or default bill to address.