You can trim a video either on the 'Edit Scene' modal or the 'Advanced Editing' screen.
Step 1: Select the video that you want to trim.
Step 2: On the right panel, click 'Trim Video.'
Step 3: Use the sliders to trim the video the way you want.
Step 4: Use the 'Play' button to preview the trimmed video.
Step 5: Click 'Done.'
Here is a short video to take you through the process: