Download, install, and run VLC Media Player. (See download link below)
Open your video file by going to Media > Open File… and browsing for your file. Or, by just dragging and dropping your video onto the VLC player.
Choose Tools from the Menu bar and select Effects and Filters.
On the Video Effects tab, tick the Transform checkbox and choose your degrees of rotation. The video is rotated counter-clockwise, so to rotate clockwise 90 degrees you’ll want to choose Rotate by 270 degrees.
Now you can enjoy your video the way it was intended to be viewed.