Click on "RDP port". The value currently in used is displayed, here by default it is the 3389 port that is used :
Enter the new port number in the corresponding window. Make sure that the port you entered is not currently used by another application to avoid any conflict, if a conflict occur Bsoft-App will not work. Here is a non-exhaustive list of TCP port that might be used by an application on your server.
A reboot of the server is mandatory for the changes to apply. 