If you haven't published your video yet, tap Edit in the top right corner.
Tap Change Sound Volume from Videos Selected in Step 1.
Select the clip you want to edit.
Use the slider to adjust the sound mix.
Tap Save again to generate a new preview.
If you're video has already been published, find it under My Account and tap on the ellipsis icon.
From here, follow the same steps to adjust the sound levels and upload a new copy of your video.