When you use your Maps site on your mobile device, the legend is located in the top right corner of the screen.
To access the legend, touch the white and blue 'layers' icon in the upper right hand corner of the screen. The legend will open from the right side of your screen like this:
Touch the green/grey dot beside each layer (or label) to turn it on or off. To change base layers, touch the white select box at the bottom of the legend area and select your preferred base layer.
To close the legend, simply touch outside of the legend area.