Although it is always best to position your camera(s) in a location where Zooming is not needed, we know that that is not always possible to achieve. You may need to Zoom in on a subject to get a proper close-up, or just to frame the shot better.
The Zoom slider is enabled by default on the live or preview window.
It is a responsive Zoom meaning the Zoom speed will depend on how you fast you slide up or down on the slider.
You can toggle this slider on and off from the Camera Control Menu.