Step 1 - Go to invoice report and find order to update
Step 2 - Click the Edit link and look for Update Supplier link on far right
Step 3 - Select the correct Supplier and enter the suppliers invoice number. if the supplier is you, then simply enter your own invoice number in the appropriate box.
Step 4 - Final Step. Click the appropriate link on the result page to update all or some of the lineitems of the order in question.