When you have located a Port Call, click it and the details will show in a panel (so called "drawer") on the right hand side. Details can currently not be maintained directly in the fields.
At the top of the drawer you can locate an action menu with several action options:
It is possible to change the instructed speed for each port call. This is a choice limited to the vessels speed and consumption combinations only depending on the leg being in ballast or laden.
The dropdown shows both the speed and consumption
Each speed will only affect the port call (leg) in question.
To add a speed for a vessel see the .