To use unscreen, upload a video (if you used an iPhone you will need to convert it to an MP4). They have a very helpful link to explain it all. They recommend using a site known as Zamzar I used the site and it worked great.
When ready, upload the video and the background will be removed after a brief time. You can then change the background and download and/or share it.
Embedded below is a 1 minute and 40 second YouTube tutorial.