You can drag a new floor from your File Browser and drag it onto the other floor. You will get a hooked shaped arrow. Let go of the mouse and it will swap the floors.
Or you can select the floor and go to the Properties tab. Down at the bottom you will see a Media section. Hit on the To button and you can choose to swap it with another clip in your projects.
Hope this helps and best of luck.
Utah Video Productions
Check out my Motion Training DVD
Check out my Motion Tutorials