From the manual:
Camera Zoom
Optical/Digital zoom: For cameras which support optical or digital zoom but cannot be adjusted using the RC’s right wheel: hold the RC’s Fn/C2 button and use the left RC wheel to adjust the zoom.
Custom Function: It is also possible to control the zoom (optical or digital) using the custom functions “Zoom In” and “Zoom Out”. Only for cameras which support zoom in the DJI firmware.