If you have configured the SysManSMS Server to use a GSM Modem on a specific COM port and need to change the port, you will have to un-install and then re-install the software.

A quick workaround will be to edit the servers init file

\SysManSMS\GSMserver\SysManSMS.ini

with a Notepad carefully change the COM port number and then re-start SysManSMS Server.