Hi, I am making this arduino http://arduino.cc/en/Main/ArduinoBoardSerialSingleSided3 at home. Is it possible to change the port to usb. Thanks
No. The whole POINT of building an RS232 SERIAL Arduino is that you can connect it to an ancient PC that has a hardware serial port.
If you want to use USB to talk to an RS232 serial Arduino you will need to by a USB-to-RS232 adapter such as Amazon.com : USB RS232