As shown in video 10

Add this code to the bottom of your active themes functions.php file
or better yet add it to the child themes function.php file 
or if those are not yet possible then use a plugin called Code Snippets as demonstrated in the video

https://wordpress.org/plugins/code-snippets/

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// customize the login error message

function login_error_override()
{
    return 'The user name and password is incorrect.';
}
add_filter('login_errors', 'login_error_override');
