If you have the .hex file. One way is probably to download the Arduino IDE.
It can't upload hex files itself, however its packaged with AVR tools with which you can upload the hex file. However this needs a ISP, you can't use the USB for this.
I just found this (never tried ) but it looks like a good alternative Arduino-XLoader It will allow you to use the USB.
However if you have the human readable code ( .h &.cpp, not .hex ) then you can use the IDE very easily, and also modify the code to your configuration.
IDE can be found here: http://arduino.cc/en/Main/Software