Adjust in the playback screen
1

Tap the screen and press the settings button.

2

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Tap the Aspect Ratio button in the Video tab.

3

There is a wide range of ratios that you can select.

Adjust in Settings
1

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Tap Settings at the bottom and select Video.

2

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Select Aspect Ratio.

3

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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.