In Eagle, click on the Manufacturing tab on the right of the board editor interface to preview the PCB.
Use the dropdown menu on the bottom left of the PCB Preview pop up window to toggle between the top and bottom of the PCB.
Click on the gear icon in the top left of the PCB Preview pop up window to show a menu for customizing the PCB.
In the the list you can set colors for the PCB's background, copper, silkscreen, solder mask, and substrate.
Top & Bottom Labels
Click on the Layer Settings icon in the toolbar on the left to open the Visible Layers panel.
Layers 25-28 contain values that will appear on the top and bottom of the PCB. If you'd like to remove them, you can use the delete tool to delete the labels from the top and bottom of the PCB.
tPlace & bPlace Layers
In the Visible Layers panel, show layers 21 (Top) and 22 (Bottom).
Elements on these layers are commonly displayed on the top and bottom silkscreen of the PCB.