yoast_seo_meta_sync_indexable.php

<?php
/********* DO NOT COPY THE PARTS ABOVE THIS LINE *********/

/* Sync post meta to Yoast Indexable
 * Credit: Yoast team
 * Last Tested: Dec 20 2022 using Yoast SEO 19.13 on WordPress 6.1.1
 */

// The ID of the post to modify
$post_id = 1;

// The post meta to modify
update_post_meta( $post_id, $key = '_yoast_wpseo_focuskw', $value = 'Custom focus keyphrase' );
update_post_meta( $post_id, $key = '_yoast_wpseo_metadesc', $value = 'Custom description' );
update_post_meta( $post_id, $key = '_yoast_wpseo_title', $value = 'Custom title' );

// Update post to sync from post meta to yoast indexable
$my_post = array(
	'ID'           => $post_id
);

wp_update_post( $my_post );