wp_enquire_script(‘jquery’) – cutom location

Add the following line in functions.php

function custom_init_method() {
wp_deregister_script( 'jquery' );
wp_register_script( 'jquery', 'http://ajax.googleapis.com/ajax/libs/jquery/1.5/jquery.min.js');
wp_enqueue_script( 'jquery' );
}

add_action('init', 'custom_init_method');

and add the following line on header.php before wp_head();

wp_enqueue_script('jquery');