To change your profile picture, first go to your profile. Now press on the "Pencil" that is located below your profile picture. A window will then appear in which you can choose between "Take photo" and "Import from library".
If you choose "Take photo", your camera will open and you can take a photo as you wish. If you are satisfied with the result, press "Use photo". If you want to take a new picture, press "New".
If you choose "Import from library", your photo library will open and you can choose a picture.