In order to change the code, it needs to receive a new code through X10 - use your PC software to do that - just send it an on and off with the device control you want it to use. Thats the easy part.
the hard bit is having it pick up the new code. For this, you need to put it into program mode - you need to connect it to a swicth, and switch it off and on quickly 4 or 5 times until it no longer swiches on or off - its now in program mode. Use the PC software to now send the X10 on and off with the new X10 device code, and it will change to the new specified device code.