You can certainly change a video project's aspect ratio by clicking on the button in the lower right of the Preview window.
However, if you've got video you shot from a phone held upright and you want to change it to widescreen video, there's no real way to do this. You have a choice of either enlarging the video so that it fills the widescreen video frame, losing the top and bottom of your video, or you can fill the frame with fogged video. I show you how to do this in this video. Although the tutorial is for another Adobe product, the principle is the same.