1. Login to your iOS Switch Mobile App and click on the Profile Screen, the fifth icon on the bottom navigation bar and is represented by the three stacked lines 
icon.
2. Click on the Settings icon in the upper right hand corner of the profile screen which looks like a gear.
3. Toggle the "Enable Face ID" slide to the green position to turn it on.
4. You are done! You have enabled Face ID on your Switch Mobile App.