Not much to know.. There's a wheel on the top right side of the controller. Move it one way to zoom in and the other way to zoom out. You can see the focal length changing on your monitor as things appear to come closer, or move further away.
Also. In DJI Go, the current focal length is displayed on the screen. There's a bar (next to the gimbal position bar). It will show a scale from 24 to 48mm when in in photo mode and in video modes from 1080P/60 frame and higher.
If you're in 1080P/30 frame video or lower, you get lossless digital zoom and the focal length indicator bar will show from 24mm to 96mm.