You can change your password by visiting the login screen in the app. Here you must input your email address. You'll then be sent an email, please press the button in the email to reset your password and follow the steps to create a new password.
If you still have access to your account and wish to change your password, this option is available by pressing the account button on the top left of the home screen, then account options, then password. You will still need to know your previous password for this method. This option will not show if you've used a Facebook account to sign up with Yomo.