In the Social Studio editor, there is a panel at far left with 3 menu options: Editor, Change Video and Change Music.
If you click "Change Music" in the Editor/Video/Music panel or click "Replace Audio" at bottom right, next to the music timeline, you will be able to swap out the current soundtrack.
You will be redirected to the music library. From here you can select from 1000s of soundtracks. Click any Genre in the library to see all tracks in that genre. You can preview any track by hovering over it. When you find one that you like, click "Use Track" and it will be added to your project in place of the current music.
You will then be redirected to the editor screen where you can preview your project with the new music.