How to change a collection field value with presave
This code worked for me need to add  in presave hook  
//field collection field  
  $item = $entity->get('field_items')->value;

$fc = FieldCollectionItem::load($item);

foreach ($fc as $fckey => $field) { 
    if ($fckey == 'field_name') {
      $value = $field->getValue(); 
      $value = strip_tags($value[0]['value']);
      $summary_value = $entity->get('field_fc_child')->value;
      $entity->set('field_fc_child', $value);
    }
  }