Thanks Christoph for your reply.
I am looking for a method that updates the firmware on the ATMega8U2 with out the need for DFU bootloader.
But, the procedure in the link http://arduino.cc/en/Hacking/DFUProgramming8U2
needs the DFU bootloader.
and also, the procedure in the above link doesn't use any external programmer like USBTinyISP.
Could you or anyone else please provide me the details on how to upload the firmware on the ATMega8U2 with no DFU bootloader on it using external programmer like USBTinyISP.