Update cart line item using 'change' method and line items key

shopify-cart-change.liquid

<script>
  let productObject = {
    id: itemInCart.key, // important to use key of type string
    quantity: 1,
    properties: {
      'PropertyName': getCookie(PropertyName)
    }
  };
  return fetch('{{ routes.cart_change_url }}.js', {
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify(productObject) // no additional braces needed
  });
</script>