I would like to update the order records of the same Order No. by importing, why do previous records disappear?

Order record could be added in the system by importing, but note that if the orders of the same [Order No.] are imported separately, the new order data will cover the original data; that is to say the new data will replace the old one.

Therefore, if you would like to keep the old data and add new data, please import both the previous one and the new one together.