If orders were not completed online you may use order/complete_from_history method to complete orders using history messages:

order/complete_from_history&params={"itemId":<long>,
				    "orders":[uint]}

 Be sure that units is assigned to order!
Use:

• order/update with callMode=create to create order with unit assigned;
• order/update with callMode=assign to assign unit to already created order.

{
    "completed":<bool>        /* checked orders aggregated status: 
                               1 - completed (if at least one order was completed during request processing; such order status would be changed to "s":2), 
                               0 - not completed (no order was completed during request processing) */
}