How to use the 9-second short film replacement soundtrack?

1 We first in the mobile phone to find the inside of the "Together"-"my" and then we click on "I joined the crowd" as shown in the picture.

2 After we click on the "I initiated" muscle, you will see a "short film" we click Into.

3 then we will press the "red button" to take a short film, and then click "Next". (pictured below)

4 then in the short film production process we can click on the "soundtrack" after we click on a "next" in this said can be written a word "confirm" so that we can replace their own oh.