From the My Videos screen find the video for which you want to change the music and tap the Gear icon.
Then tap the Edit button.
Tap Change Music Selection and Music Mix Volume.
After tapping on Theme Music, locate the song you want and tap on the name of the song.
Move the slider to set the mix - to the left to make music louder and to the right to make music softer, then tap Save.
If you’re done making changes to this video, tap Save.
Then tap Save again to give your video an updated title, then tap Save to let it process and upload.