Go into Create Mode (top of app)
Click on the Create button (bottom left)
Follow important instructions on taking selfie, in particular
neutral expression (no smile)
even lighting (no shadows)
face the camera
don't block your facial features (hair, glasses, etc.)
Position your face inside the outline
Choose from one of the 4 avatars that most resembles you.
It is easy to edit the avatars (change hairstyle, remove/add glasses/change clothing)
If you prefer, you can hit the "infinity" icon to generate 4 more avatars, or "X" to take another photo.
Go to Create Mode (top of app)
On the bottom, click and hold on the avatar you want to delete until it shows an "x" marker
Click on the "x" and the avatar will be deleted. You will see that the avatar bar will reflect the deletion.