From DeviceManager, select "View devices by type". Then open "Ports (COM & LPT)" to find out the Com Port number.
Changing the COM Port Number
From DeviceManager, select "View devices by type" à "Ports (COM & LPT)". Select the USB serial port and click Properties. Select the "Port Settings" tab. Then click Advanced. Choose the required COM port number from the list and click OK.
Notes
If a modem is attached to the USB serial port when the port number is changed, it will continue to work under the new port number.
If a modem had been attached to the USB serial port but was not attached when the port number was changed, it will not work when re-attached under the new port number. In this case, the modem must be removed from the system and re-installed.
In general, if there are devices which have been attached previously to the USB serial port, they may not work after the port number has been changed. If this is true, remove the device from the system and re-install it.