If you want to change the WordPress dashboard login logo then this piece of code can meet your requirement.

Just paste this code in the your theme functions.php

function custom_login_logo() {
    echo '<style type="text/css">
        h1 a { background-image:url('.get_bloginfo('template_directory').'/images/custom-login-logo.png) !important; }
    </style>';
}

add_action('login_head', 'custom_login_logo');