How do i change page template in bulk?

Try something like this:

        $pages = get_posts();

        $page_ids_to_swap_the_template = array( 1, 2, 3, etc... );

        foreach ( $pages as $page ) {
            if ( in_array( $page->ID, $page_ids_to_swap_the_template ) ) {
                update_post_meta( $page->ID, '_wp_page_template', 'new_template.php' );
            }
        }