Newby Question: How to flash the I/O-Controller

Hi all,
maybe this is a stupid question but i did not find any info yet:
Pixhawk and PX4FMU/PX4IO contain each a main microcontroller STM32F407 and a small auxiliary controller.

Question: How is the firmware of the auxiliary controller updated? I see anywhere only manuals how to upgrade the main controller. Is the aux-controller flashed via the main or how is this handled?

Please help?