Hello there. Thank you for reaching out and for the inquiry. VanzPix is correct on post #2 and I am sorry to say that the music on the DJI Mimo application from the DJI Osmo Mobile 3 can not be changed. Thank you and keep on filming.
I figured a way how to this. In edit mode turn the volume down all the way and save it. Using another phone editor like iMovie you can add new music and save it again.