System doesn't allow you to modify the contents in a shipment that is In transit. If, for some reason, you have to edit the contents of a shipment after it is sent, you can edit the shipment, update its contents and send the shipment again.
Navigate to Manage shipments menu from Tasks section and select the desired shipment whose contents should be edited
Once a shipment (which is In transit) is selected, system disables buttons related to actions that are not applicable/permitted on selected shipment. Now, select Edit
System asks you for confirmation. Select Yes
Status of the selected shipment is changed from In transit to Not shipped and you may update the contents of the shipment. You may add additional specimens or remove existing ones.
Once you have made necessary updates, re-send the shipment.