To remove the background of a video:
Step 1: Select the video.
Step 2: Click on the Effects option on the top panel.
Step 3: On the left panel, select the Remove Background option.
Step 4: Confirm your action, then give it a minute or so and the video background should be removed.
You can then place the Video on another background.
Adjusting the layer order to bring a new background below the existing video will also help.
All videos whose background has been removed will be saved in your project media folder for use anytime in the future, even with other projects!
Note: As this feature is still in beta, it may not work as expected at times or take slightly longer than expected. Currently, this feature only works for uploaded videos in the mp4 format that are below 5 minutes, and not on stock footage.