Click the tab at the top that says ‘Sky & Fog’
Then you will see a large ball to the right… on that ball is a representation of the sun’s position.
Click on the ball and drag to move the sun.
Also if you go into the sky lab (double click on that ball) and choose the Sun tab, you have control over the sun’s position size and brightness.
Click on Sky & Fog on the shelf as TheSavage64 said. Then click on the small cloud with the rainbow. This opens the Sky Lab which gives you control over the sky and you can set the sun’s position numerically, if you prefer.