Here’s how to change the default face in the app, or add a new face:

1) Tap on the "active" face at the bottom of the screen. This will take you to your face library.
2) Choose the face you wish to use.
3) You can also create a new face from this menu by tapping the "plus" button.

Once you tap the "plus" button, you can either take a selfie, or click the square icon to the left of the camera button to select a photo from your camera roll.