In the Layout section of the bottom toolbar, tap on this icon to see all layers:
Each Eagle file is compiled of various layers like the top and bottom copper layers, the silk screen and many more. Copper will list all layers (but hiding those that don't have any content) in the layer screen. In this screen you can activate or deactivate each layer separately. If you want to hide all bottom layers, there is a faster way to do that: X-Ray.