It's not complicated. I did an upgrade to my newly arrived P4 yesterday via my iPad. Starting with the tablet connected to the controller, and the controller and aircraft powered on, 'clicking' the upgrade request in ;DJI-Go' starts the tablet downloading the new firmware. When that is done, it askes you to unplug the tablet's USB cable from the controller and reconnect it to the aircraft using the supplied USB to micro-USB adaptor. When this is done and the Go app 'sees' the aircraft connected by USB, it starts the firmware upgrade, with a progress bar. When all done, reconnect the tablet to the controller, re-boot the controller and aircraft, and all is done.
David