If you have logged in with Social Media, your avatar will be pulled through from your account. You can find this in the top left of the screen. If you have logged in with an email and password you will have the blue account icon as show below. Click the icon and your menu will appear.
Choose "Settings" and you will be taken to your account settings.
It is from this page that you can change your password and update your username. Please note, your email remains the same as when you signed up for your account.
To change you password, enter your current password then enter and re-enter a new one. Click on "Update Profile" to change your password.