Exchange the WordPress backend dashboard logo in the top left corner (should be 20px by 20px) through functions.php action hook

listing-18-11.php

function wh_wordpress_backend_custom_logo() {
  echo '
  <style type="text/css">
  #wpadminbar #wp-admin-bar-wp-logo > .ab-item .ab-icon:before {
    background-image: url(' . get_bloginfo( 'stylesheet_directory' ) . '/images/custom-logo.png) !important;
    background-size: 20px 20px;
    background-position: 0 0;
    color:rgba(0, 0, 0, 0);
  }
  #wpadminbar #wp-admin-bar-wp-logo.hover > .ab-item .ab-icon {
    background-position: 0 0;
  }
  </style>
  ';
}
add_action( 'wp_before_admin_bar_render', 'wh_wordpress_backend_custom_logo' );