lift the rewind crank and the back opens.
The button on the bottom is for rewinding at the end - push it in and it releases the gearing so you can use the crank to rewind.
Not too long ago in this forum had a question on how to "rewind" the film. Can you believe it? Got the film in, took the pictures, couldn't rewind the film. Don't remember which model, but I noticed your ME is just like my Spotmatic for rewind.