You can use multiple parts of the same video by trimming the video. You can do this either on the 'Quick Edit' screen or the 'Advanced Editing' screen using the same process.
Step 1: Select your video.
Step 2: On the right panel, click 'Trim Video.'
Step 3: If a video is 10 seconds long and you want to use the first and the last 2 seconds of the video. Drag the slider to accommodate for the first 2 seconds.
Step 4: Add a section by clicking 'Add Section.'
Step 5: Drag the sliders for the new section to accommodate for the last 2 seconds.
Step 6: Click 'Done.'
Here is a video to take you through the process