Your username is changed in the app under Profile: tap the pencil on the picture, enter a new name, save. It may be 3 to 20 characters long and consist of lowercase letters, digits and the underscore.
You change your email address in the app under Profile → Email address. Enter the new address and your current password; we send a confirmation link to the new address. Your address changes only once you open that link — until then you keep signing in with the old one. The link is valid for one hour. A notice also goes to the old address, so no change goes unnoticed; if you no longer have access to it, that is no obstacle. If you would rather start over entirely, you can delete your account (Delete your account).
Your password is changed under Profile → Change password while you are signed in, or via Reset your password if you are not.
There is deliberately no profile picture: the image in your profile is your initials.