You can replace a project's video by clicking "Replace project video" in the Project Dropdown Menu.
This will directly replace the video for the project with the new video.
If your new video has different edits to the old video, all captions will need retiming.
If the video is identically timed, and differs only visually, you shouldn't encounter any problems.
The purpose of this feature is to allow for updating a video with a new version that has had its visuals - but not edits or sound - revised. If the video is entirely different, we recommend creating a new project for it.