Can’t update multiple rows with $wpdb query

$wpdb->query() returns number of rows affected by the query. It’s returning NULL indicates that there is an error in your query.

The problem with your query is that you are updating $wpdb->posts table, but the condition depends on $wpdb->postmeta table. You have to join these tables for the query to work.

Another problem is that you have to place the post inside quote

$wpdb->posts.post_type="post"

Try the following query

global $wpdb;
$result = $wpdb->query("UPDATE $wpdb->posts
                        INNER JOIN $wpdb->postmeta
                            ON $wpdb->posts.ID = $wpdb->postmeta.post_id
                        SET $wpdb->posts.post_type="editorial_post"
                        WHERE $wpdb->posts.post_type="post"
                            AND $wpdb->postmeta.meta_key = 'tag'
                            AND $wpdb->postmeta.meta_value="editorial"");
var_dump($result);