<?php
  wp_deregister_script('jquery');
  wp_register_script('jquery', 'http://ajax.googleapis.com/ajax/libs/jquery/1.4.0/jquery.min.js', false, '1.4.0');
  wp_enqueue_script('jquery');
?>
<?php wp_head(); ?>

http://www.prolificnotion.co.uk/adding-jquery-to-a-custom-wordpress-theme/

Place the following code in header.php before the call to wp_head().

(If you do not include the exact version number when calling libraries from Google's AJAX Libraries API, a redirect is used to pull up the most current version. While this is handy, it prevents browsers from caching the script: effectively negating one of the major reasons for using Google's AJAX Libaries API.)

Load jQuery from Google AJAX Libraries API in Wordpress

google, wordpress, jquery

PHP