Joe has it right. Turn the right-side key as though removing the back and that allows the film to be rewound. The key doesn't have to be turned more than roughly 30 degrees to activate the rewind function.
When you have the film rewound and remove the back, take note of the mechanism that depresses the rewind pin inside the camera's body. It's a rather ingenious set-up and seems to work well. Still, I've never been fond of it, preferring an external pin that I can see....... but that's just me. 🙂
Walker