wp-cat-postmeta.php

<?php
/* Update postdate for all post in a category */
// Get IDs of all posts in a category.
$args = array(
    'category_name' => 'cat-name',
    'fields' => 'ids',
);
$query = new WP_Query($args);

// Update meta for all IDs.
$meta_id = 0;
if ( $query->have_posts() ) {
	foreach( $query->posts as $post_id ) {
		$meta_id = update_post_meta( $post_id, 'meta-key-name', 'Meta Value');
		// Print ID and returns.
		// echo "$post_id - $meta_id\n"; 
	}
}
?>