I need to know how to turn a video from vertical to horizontal shot with EOS 5D MarkII

[Ger_a_rdo]

I just started shooting video with my EOS 5D Mark II. A portion of such video I turned my camera to the vertical position. When I downloaded the video, that portion showed as vertical! I can shoot video using my iPhone 6 and turn it both ways and the video will correct itself to the position that the phone has. How come a camera such as mine is limited by the position of it? Is there a way to correct this? Do I need a special software?

Any assistance with this subject will be greatly appreciated.

Thank you kindly!

[Waddizzle]

How come?  Because it is a camera, not a telephone. 

Auto-rotation of the image is not a desireable feature in a camera because it constrains creativity, IMHO.