How do you take an unfinished film out of a camera and not ruin it, to then put it in another camera and use?

Other than as Sincerity says ... If the camera has a manual rewind, use that to rewind the film.  Then take the film out of the "old" camera, and put it in the "new" one.  Then WITH THE LENS CAP ON, take an equal number of pictures - OR - some cameras will allow you to manually advance the film.