Adjust in the playback screen
Tap the screen and press the settings button.
Tap the Aspect Ratio button in the Video tab.
There is a wide range of ratios that you can select.
Adjust in Settings
Tap Settings at the bottom and select Video.
Select Aspect Ratio.
Select the screen aspect ratio you want.
The adjusted screen aspect ratio will be applied every time you start playback and you can readjust it in the playback screen.