我没有找到任何可用于自定义消息的挂钩。所以决定删除原来的更新nag，并将我们的自定义nag放在那里。

让我们先删除原始nag

// Admin menu hook
add_action( \'admin_menu\', \'remove_core_update_nag\', 2 );

/**
 * Remove the original update nag
 */
function remove_core_update_nag() {
    remove_action( \'admin_notices\', \'update_nag\', 3 );
    remove_action( \'network_admin_notices\', \'update_nag\', 3 );
}

// Admin notice hook
add_action( \'admin_notices\', \'custom_update_nag\', 99 );
add_action( \'network_admin_notices\', \'custom_update_nag\', 99 );

/**
 * Custom update nag
 */
function custom_update_nag() {
    if ( is_multisite() && !current_user_can(\'update_core\') )
        return false;

    global $pagenow;

    if ( \'update-core.php\' == $pagenow )
        return;

    $cur = get_preferred_from_update_core();

    if ( ! isset( $cur->response ) || $cur->response != \'upgrade\' )
        return false;

    if ( current_user_can(\'update_core\') ) {
        $msg = sprintf( __(\'<a href="http://codex.wordpress.org/Version_%1$s">WordPress %1$s</a> is available! <a href="%2$s">Please contact now</a>.\'), $cur->current, \'your_custom_url\' );
    } else {
        $msg = sprintf( __(\'<a href="http://codex.wordpress.org/Version_%1$s">WordPress %1$s</a> is available! Please notify the site administrator.\'), $cur->current );
    }

    echo "<div class=\'update-nag\'>$msg</div>";
}