There is a no wp_update_tags in WordPress codex.But we realize it through an indirect method.Accordingly, the function below; first, remove the tags related with post_ID and insert new tags.

// Coded by tankado at tankado dot com
function wp_update_post_tags($post_ID, $tags)
global $wpdb;
// Delete related tags from database
$sql = "DELETE FROM wp_term_relationships ";
$sql .= "WHERE wp_term_relationships.object_id = '$post_ID' ";
$sql .= "AND wp_term_relationships.term_taxonomy_id = wp_term_taxonomy.term_taxonomy_id ";
$sql .= "AND wp_term_taxonomy.taxonomy = 'post_tag'";
$wpdb->query( $sql );
// Insert new tags to database
wp_set_post_tags($post_ID, $tags, false );