Use a video editor with background removal or chroma key
Open the video in the editor
Select the background removal, cutout, or keying tool
If using a green screen, apply chroma key and choose the background color
Adjust tolerance, edge, and spill settings until the background is removed cleanly
If no green screen is available, use AI background removal or rotoscoping tools
Refine the subject mask frame by frame if needed
Add a new background or leave it transparent
Export the video in the desired format