How to put the CUAV Pixhack V3 in DFU mode to upgrade bootloader?


I hope someone from CAUV can answer this question about the CUAV Pixhack V3 autopilot.

The problem is that the new Ardupilot firmware requires to flash a new bootloader replacing the original bootloader.
There is a procedure to do this with Ardupilot which is very well documented but it requires to put the CUAV board in “DFU mode”. Normally there should be a button to press while plugging the USB cable on mission planner which would put the board in DFU mode in order to reprogram its bootloader on the StM32F4 proc.
But there is no such button on the CUAV board.

So the question is : how can I put the board in DFU mode ?

Without a solution, that means the CUAV V3 board is unusable with the latest Ardupilot firmwares which I cannot believe ?

Thanks in advance