PX4 IO source code

Hi guys
i wan to programPX4 IO where can i find the source code and how can i upload this ?
thanks

modules/PX4Firmware/src/Drivers/Px4io

thanks for replying
how can i upload this frimware?

I’m not sure for that.
But I guess px4io get it from fmu when boot up

instructions on building the code is on developer links on http://ardupilot.org/dev/index.html