The Keurig was designed to stay on all the time, but I think most people (myself included) do turn them off unless they afe making a lot of coffee throughout the day.
On your screen, there should be a circular icon in the lower right. Just tap it, and that will turn it off (this is the same thing you press to turn the machine on).
You may wish to go into the settings, and turn on the energy saver. Thiswill turn the machine off automatically after a predetermined period of sitting idle. for instance, we have our machine set to turn off after two hours. This gets us through the morning until we generally stop making coffee for the day.