You can choose to use either the Actual Port Cost or Preliminary Port Cost of previous port calls to update the port cost of the selected port call.
To make use of a Preliminary Port Cost of a port call on the system, click the button to the right of the Preliminary column. To make use of an Actual Port Cost of a port call on the system , click the button to the right of the Actual column. Clicking this button will also update the currency of the port cost of the selected port call to the currency shown on the row.
Clicking any of the "Use" button will update the port call's port cost, which will be accumulated on the voyage's overall port costs.