You can change the order of your scenes either on the 'Storyboard' screen or the 'Advanced Editing' screen.
On the 'Storyboard' screen, use the arrows on the left-hand side of every scene to move the scene up or down in the video.
On the 'Advanced Editing' screen, use the timeline to drag the scene and place it in its new position.
It's that simple!
Here is a small video explaining the process.