Using SMD Component in Eagle
You have (at least) two options:


If the library you are using already contains BC548 in SMD, you can just right-click on the transistor you want to switch to SMD and click "Replace" if SMD transistor is a separate device, or "Package" if it is just a package variant of a device you are using.
If there is no suitable package in your library, you can find another one, or add a package yourself. To do so, right click on the transistor in the schematic, and choose "Open device". You will find a nice tutorial on editing libraries here: https://learn.adafruit.com/eagle-tutorial-how-to-add-a-new-package-to-a-component/. You will also find many ready to use libraries on Cadsoft site here: http://www.cadsoftusa.com/downloads/libraries. Personally, I recommend ic-packages_big-pack_v1.0.rar library. It contains most popular packages that you can reuse in your own designs.