Eagle, by default, puts all the components on the PCB’s top layer, usually displayed with red color:

Choose the Mirror tool:

and click on your component: Eagle will move it on the bottom layer, displayed with the blu color:

Eagle, by default, puts all the components on the PCB’s top layer, usually displayed with red color:

Choose the Mirror tool:

and click on your component: Eagle will move it on the bottom layer, displayed with the blu color:

… or just click the Middle Mouse Button