<?php
$args = array (
'posts_per_page' => '-1', // all posts
'post_type' => 'your-post-type'
);
global $post;
query_posts($args);
while ( have_posts() ) : the_post();
update_post_meta( $post->ID, 'field_slug', '' );
update_post_meta( $post->ID, '_field_slug', 'field_key' );
endwhile;
wp_reset_query();
?>