By default , the port is not exposed to the user to change, however it is possible to add / change the required port in southbound start.bat/sh file.
Look below the file location in the dx gateway zip. This is the zip which have been download from DX SAAS portal , this is called dx gateway which has to run in vm/ host system in order to use bidirectional updates.
Add the entry '--server.port=<required port number >'' like below in the bat/sh file.
Like Port number 8080 has added as per below example. This variable wont be available initially. For already configured dx-gateway/southbound gateway restart of service or uninstall /install option is required .