Very HOT water and vinegar. 3cups to gal of water. If you have vinyl paper, sometimes the top face(vinyl) will peel off dry. If not, score it with a Paper Tiger. If doing a large area, put water/vinegar in a garden sprayer. Wet wall, wait 20 mins. wet again, wait another 15-20 mins (let the water soak thru to the glue) Should scrape off easily, if not, re-wet again.
Good luck