First we need to download the source code from HERE

You can find assembly language files along with hex and lst files.  

If you want to modify anyting then you need to use the assembler .

I have used the C51ASM assembler for Windows from Microchip from HERE

C51ASM is a 2 pass assembler same as metalink and produces same checksum without any errors or warning.

To program the hex file into the FLASH of the target microcontroller, we need the WCHISPToolwhich is available .

We can easily program using USB port which supplies VCC, GND and D+ and D-