Pete, when you look at the camera from the front- the lens is surrounded by a large serrated ring which is the lens locking collar. When the lens is on, the large ring has a red dot on the right side .YOU MUST WIND THE FILM ADVANCE BEFORE REMOVING LENS! To remove the lens Push the lever on the right of the camera as you look at it and turn the lens locking collar counter-clockwise until the red index mark is in a vertical position and pull the lens straight out of the body. Now with that lens off make sure that the new lens on the rear of the lens that the two pins are aligned with the red marks. Then the lens can be mounted. If you fired the shutter button you must wind the film advance first Always to mount or dismount the lens.
Edit: You beat me to it Pete. Good for you Good luck with the camera they are marvelous cameras and great glass!