How to alter existing metatag?

You might be better off with the Schema.org Blueprints module → , it's much more flexible for handling complex or nested data structures. As it is, the Schema Metatag module is not going to be expanded to cover this scenario.

FYI hook_metatags_alter() is the hook you want.

Cool thanks a lot! I used hook_metatags_alter() to that purpose. For everyone needing the same / similar solution I will just leave the code I wrote here, maybe it will help someone.

<?php

/**
 * Implements hook_metatags_alter().
 */
function YOURMODULE_metatags_alter(array &$metatags, array $context) {
  
	if (isset($context["entity"]->product_id)) {
		
		$product_id = $context["entity"]->product_id->value;
		
		$review_ids = \Drupal::entityQuery('comment')
		   ->condition('entity_id', $product_id)
		   ->execute();
		   
		// Blank metadata
		$metatagData = [];   
		$metatagReviews = [];
		
		if(!empty($review_ids)) {
			
			$reviews = \Drupal::entityTypeManager()->getStorage('comment')->loadMultiple($review_ids);
			
			$numberOfRatings = 0;
			$totalRating = 0;
			
			foreach($reviews as $review) {
				
				// 0 - 100
				$rating = $review->field_fivestar->getString();
				
				$totalRating += $rating;
				
				$numberOfRatings++;
				
				if ($numberOfRatings < 10) {
					
					$reviewData = [];
					$reviewData["@type"] = "Review";
					
					$reviewData["datePublished"] = \Drupal::service('date.formatter')->format($review->created->value, 'custom', 'Y-m-d');
					
					$reviewData["name"] = $review->comment_body->value;
					
					if (strlen($review->comment_body->value) > 80) {
						$reviewData["name"] = substr($review->comment_body->value, 0, 80) . '...';
					}
					
					$reviewData["reviewBody"] = $review->comment_body->value;
						
						$author = [];
						$author["@type"] = "Person";
						$author["name"] = $review->getOwner()->name->getString();
						
					$reviewData["author"] = $author;
					
						$reviewRating = [];
						$reviewRating["@type"] = "Rating";
						$reviewRating["ratingValue"] = $rating / 2 / 10;
						$reviewRating["bestRating"] = "5";
						$reviewRating["worstRating"] = "1";
					
					$reviewData["reviewRating"] = $reviewRating;
					
					array_push($metatagReviews, $reviewData);
					
					
				}
				
			}
			
			$metatagData = unserialize($metatags["schema_product_aggregate_rating"]);
			
			$calculatedRating = $totalRating / $numberOfRatings;
			
			$metatagData["ratingValue"] = $calculatedRating / 2 / 10; // 80 / 2 / 10 = 4 stars
			
		}
		
		$metatags["schema_product_aggregate_rating"] = serialize($metatagData);
		$metatags["schema_product_review"] = serialize($metatagReviews);
		
	}
	
	// \Drupal::logger('my_module')->notice('<pre>&#039; . print_r($metatags, TRUE) . &#039;</pre>');
	
}

Yes I know this is not the ideal solution but it works for now.

Automatically closed - issue fixed for 2 weeks with no activity.