If you have the Arduino IDE, there is a command line utility built into the package. it is called "AVRDude" and can be executed from a terminal window. Here is the command line that works with my setup.
avrdude -pm328p -carduino -P/dev/ttyUSB0 -b57600 -Uflash:w:ubitx_CEC1.01.hex:iThe filename you want to load (.hex file) is just after the 'w:' I execute this line after navigating to the file location directory/file in the terminal program. The help function for avrdude explains all the switches that can be invoked.
Hope this helps.