Hello sir, you could download DJI assistant 2 and install it according to the operating system of your PC, then connect your aircraft to your PC, run the assistant, you will see " Factory reset" on the screen.
Please download it here: http://www.dji.com/phantom-4/info#downloads
I usually backup files on Dji along with my personal settings to computer, in case of losing data some day. It's necessary to me for these video files and photo files are almost everything to me. And I think it's safer to save them all on my PC hard drive.
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