How do I rewind my old Super 8 spool?

I recently discovered an old super 8 projector in my grandfathers attic, and realized that there was still a super 8 cartridge in it. Once I hooked it up and turned it on, the film stayed in place. I took out the cartridge and opened it up, only to find that the film was not wrapped tightly enough around the spool to spin correctly. My question to anyone who can help at all, is how do i correct this problem? Is it possible to rewind the film manually, and if so is it safe for the film? The film cartridge says "Fairchild Moviepak Seventy 10", and to my knowledge it is 8mm film. Any help would be appreciated, as I have almost zero know how when it comes to old cameras and film.