You can only change quantity information against an order line that has already been added to DARWIN. It is not possible to change the dock code or other data.
The dock code etc comes from the message sent by your trading partner, so if this is incorrect it means that the original order was incorrect.
However, you can add a new order line to the system using the "Copy Order Line" option from the Confirmed Orders screen.
This will take a copy of the original order line received from your trading partner and allow you to change whatever details are necessary (including the dock code and any other data).
The original order line can then be deleted and the copied order line despatched in its place.