How to set aspect ratio?
1. Press on the screen while the video viewing mode to show the app's toolbar.
2. Press the Settings button in the lower left corner 
.
3. Choose
a. Content mode→Fit width to fit the video by its width.
b. Content mode→Fit height to fit the video by its height.
c. Aspect ratio→Auto / 16:9 / 4:3 to choose screen aspect ratio.
