Here's how to update the firmware:

Which goggles are compatible with DJI Phantom 3 Professional?

Install the HDMI output to the RC and reinstall thr RC firmware. Then you can use any HDMI input goggles including Dji's. I prefer Avegant video headset. Though the video is 720p and image size is much smaller comparing with Dji but you can adjust the diopter and the sharpness in hdmi is much better.

Good informative video.