The Zoom button will be located beside the Seasonal and Unworn bars at the bottom of the styling screen. Tapping the Zoom button will zoom in and out of the model while on any tab on the styling screen.
 
Auto-Zoom is ON:
If your Auto-Zoom is toggled ON, your model will automatically zoom-in when you tap on the Makeup tab while styling for challenges. Tapping out of the Makeup tab will automatically zoom-out your model as well. 

Auto-Zoom is OFF:
If your Auto-Zoom is toggled OFF in the Setting, players will need to manually Zoom. Please tap the Zoom button to zoom in and out while on any tab on the styling screen.