You can run a query for the unique_id
with the below:
$existingPost = get_posts( array(
'meta_key' => 'unique_id',
'meta_value' => '555555',
));
if($existingPost){
$my_post['ID'] = $existingPost->ID;
$post_id = wp_update_post( $my_post );
}else{
$post_id = wp_insert_post( $my_post );
}