Click on the "TOOLBOX" icon in the top left and select (WORLD-SUN).
DAY:
You can drag the Time of Day slider to set the time of day. Other properties in this category include things like Sun Rotation, Sun Intensity, Sun Color, Size, Shafts and Volumetric Scattering.
HINT: You can toggle "Animate Time of Day" if your game requires a full day and night cycle.
NIGHT:
In the Night category, you can find properties for controlling:
