There are two items that will be required to remove the rubber underlay: a heat gun and an old spatula. The heat gun is needed to melt and loosen the glue. Because the glue is under rubber, applying heat to the rubber underlay can be dangerous because of the fumes that it will produce. This is where the old spatula comes in handy. Apply a modest amount of heat to the rubber underlay, enough to loosen the glue slightly, but not enough to melt the rubber. At the same time, push the spatula under the rubber underlay and it will loosen even more. (Sharpening the edge of the spatula can speed up the process.) Once you have removed the rubber underlay, use a sharp scraper to remove the remaining bits of rubber attached to the floor. It will be a slow process, and even with the aid of the spatula and applying only a modest amount of heat, there is still the possibility of danger from fumes. Make sure that the room is properly ventilated and that you wear a mask.