How to change vertical iPhone video to Horizontal.

I recently shot a phone video vertically and realized that it was not getting a wide enough shot. So...... I just turned the phone and continued shooting, instead of starting over shooting horizontally. Well, this did not work and I have spent a couple of hours trying to turn it around but it keeps the vertical checkerboard background and then you loose all but the center of the video. Is there a quick way to fix this?

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