Hi,
For downloading a code to 8051 controller.
You need a programmer which is hardware accomplised by a software to burn the hex file to 8051.
First interface the hardware burner to computer through serial programmer or usb programmer.
You can use USB 8051 PROGRAMMER software to burn the hex file to 8051 for usb programmer method.
After that open the burner software in the computer and open the library and select particular hex file choose load file option to download to the controller.