It depends how the video is being rendered.
If your application is the one rendering the video, then you can do the rotation in code (exactly how depends on how you are rending, with openGL or with QPainter etc).
If you don't have direct access to the video frame data, then its probably done by the hardware and you have no control over the frame in your code.
In such case the solution also is not in your code, but in hardware / system configuration.
But you supplied to little information to help you any further.