When you are in the Social Studio editor, there is a panel at far left with 3 menu options: Editor, Change Video and Change Music. You can click any of these options to edit the corresponding aspect of your video. Clicking "Change Video" will redirect you to the video library where you can change out the background video. You can also trim the current video by clicking "Trim Video" at bottom right, next to the video timeline.
Once in the video library, you can scroll through videos or search for something specific (in the search bar at top center). You can also add your own clip by clicking “My Uploads” at top right.
Once you find the footage you want, click "Use Video" to open it in the trimmer. You will be prompted to drag and trim the video to the portion you want. You can use up to thirty seconds. Once you've trimmed your video, you will be returned to the editor where you can preview your project with the new footage.