QGroundControl will ask you to plug in your FMU using a USB cable. It might also ask you to first unplug it again, because it needs to enter the bootloader before it can upload firmware. A popup will appear that asks you which flight stack you want to use. We will use the PX4 flight stack. You can choose between the standard (stable) version and test/development versions, or a custom firmware binary. It is recommended to use the stable version, for now.