At the bottom of the app you can see the App Bar. There you can find the camera icon (second from left). This button will direct you to Your Videos Page.
At this page you can see all your videos. The Main Video is marked with the star in the corner.
To Rewrite any of your videos just press on it and then press the button Record in the center.
To make another video as the main one, just press on it and then press the button Star.