When you've taken all the pictures on the roll, you must rewind the film into the cartridge before you open the back. Turn the rewind knob on the bottom of the camera in the direction of the arrow marked on it. As you do this, the film counter dial on top of the camera will rotate bacwards.

When the counter dial stops turning, wind the rewind knob a few more turns to wind the film leader fully into the cartridge, then open the back and remove the cartridge. If you are in bright sunlight, shield the cartridge from direct sunlight while removing it and placing it in its container can, or leave it in the camera if you aren't loading another roll until you get into dimmer light.