To change the resolution of your video, pause the playback and observe the four icons appearing on the right side of the screen, overlapping the video. Select the second icon from the top, which resembles a crop icon. This allows you to choose among landscape, square, and portrait layouts. Landscape being compatible with many video web platforms.
When it's time to export your video, consider selecting 720p or 1080p if you're subscribed.
This process should get you exactly where you need to be with your video resolution.