Do not continue unless you have installed Mission Planner and agreed to instal the USB Drivers during the Mission Planner Install.
Instruction for installing Mission Planner are here
If you have your Fixhawk already connected to your PC or Laptop, please disconnect it.
If you have Mission Planner running on your PC or Laptop please shut Mission Planner down
Connect the USB cable from your PC to the Fixhawk, wait until the USB connection is recognized and connected by your PC or Laptop.
Make sure no error messages displayed stating that there is something wrong with your USB connection, if there are deal with the USB errors on your PC first before continuing.
Run Mission Planner.
Follow the instructions in the video below.
To view the video Full Screen click the dual arrow icon, bottom right.
In order to avoid all sorts of weird issues it is important to erase Fixhawk non-volatile memory.
In the latest versions of ArduCopter firmware the ability to erase non volatile memory using "Terminal", "setup", "erase" has been removed
There is a way around this inconvenient issue for now, valid as at March 2015.
The solution is to temporarily load AC 3.1 which a much earlier version of the firmware, run Terminal setup erase and then Update Firmware to whichever firmware version you actually intend to use.
Warning: If this is not the first use of your Fixhawk, if you have already performed PID tuning and have any settings you wish to retain then you should save those settings BEFORE performing Terminal erase and reset. so you can load those settings back without having to do all that work again from scratch. Also be aware that this will clear ALL calibrations.
The steps below are set out in a logical sequence, do not perform them in a different sequence, do not skip any steps.
Important : In order to carry out this procedure you first need to enable Advanced View in Mission Planner.
Run Mission Planner, click CONFIG/TUNING and tick the box labelled Advanced View as depicted in the screen capture below
Failure to do that will mean you will not see the advanced options available in Mission Planner and used to carry out this operation.