If you have registered with Fauna using your Netlify or Github account, and wish to change the login mechanism to email+password, you can do that by simply going to Forgot your password? and following the steps.
From that point, you will be able to login with both your email+password and Netlify/Github.
Note: Your account will still be associated with the external provider you originally registered with. To completely unlink the account and use only email to login, please open a support ticket.