In order to add more video clips to your video, simply click the icon "Add video clip or image" on the timeline.
This will take you to the step "Add videos and photos" where you can search through the built-in stock library or upload your own media assets.
How to delete a scene
In order to delete a scene, simply select the scene click the X sign. This will delete the whole scene, with all the text messages, stickers, and other assets you might have added.