If you want to test out jQuery 1.4 with your current WordPress installation, you just need to add this code into functions.php of your theme
if( !is_admin()){
wp_deregister_script('jquery');
wp_register_script('jquery', 'http://ajax.googleapis.com/ajax/libs/jquery/1/jquery.min.js', false, 'latest');
wp_enqueue_script('jquery');
}









