To change a selfie in the Waldo Photos App:
Open the Waldo Photos app and click the 3 lines in the top left of the app.
Tap on the photo of the "faces" to bring up the users and select who you want to change. Tap on the user you want to change and select "Change selfie" to either take a new selfie with Waldo's help OR you can upload a new photo by tapping the small picture in the bottom left of the screen which will take you to your camera roll where you can select a photo.
An ideal photo is recent, well-lit with a big smile. Avoid cropping, filters, shadows, blurs, glares and accessories (hats, sunglasses, etc...)