Prior to 5.10.x, to rename an attached serial port, you need to go to Managed Devices :: Devices, tick the serial port that you want to rename and click on "Rename":

Then change the name and "Save":

From 5.10.x you can now rename the serial port from within the serial port properties. The above steps are also still available.

Under Managed Devices :: Devices, click on the serial port name that you want to change (in our case ttyS2):

In the serial port properties, you can now change its name (which was not available in previous versions) and then click "Save":

NOTE: you can also use the ttyS'port number' as the 'the current port name' if the name was changed from the default.