You do not "install" subtitles like you would a software program. You simply download the file and place it in the same folder as your video file, or "hardcode" it into the video file if you plan to watch it on a device that doesn't support external files.
You might ask: why not 1080p or 4K?
: If you prefer physical media, you can buy the DVD or Blu-ray of "Spartacus: Gods of the Arena" which usually comes with subtitle options.