Add a new field collection item to an existing field collection
You can try code:

&lt;?php

use Drupal\field_collection\Entity\FieldCollectionItem;
use Drupal\node\Entity\Node;

[....]

$node = Node::load(1);

$field_collection_items = $node-&gt;field_ckt_info_child2;
$fieldCollectionList = $field_collection_items;
if (count($fieldCollectionList)) {
 //update 
  $field_collection_item = FieldCollectionItem::load($fieldCollectionList[0]-&gt;value);
}
else {
  //create new fieldconlection
  $field_collection_item = FieldCollectionItem::create(
    [
      'field_name' =&gt; 'field_ckt_info_child2',
    ]
  );
  $field_collection_item-&gt;setHostEntity($node);
  $field_collection_item-&gt;set('field_eng', 'HELLO');
  $field_collection_item-&gt;set('field_maths_ckt_child2', 'WORLD');
  $node-&gt;field_ckt_info_child2[] = ['field_collection_item' =&gt; $field_collection_item];
}

$node-&gt;save();