How to update individual line items in Shopify cart when the same product is added with multiple qua

[UbaidmIsmail]

I'm working on a Shopify store where I need to allow users to customize each cart item separately, even if they have added the same product multiple times with different quantities. For example, a customer can add a product to the cart with a quantity of 2 but would like to personalize each item (e.g., sending cakes to different people by adding From - For products might be same and in bulk quantity).

I have split the quantity in the cart visually using Liquid like this:

{%- for i in (1..item.quantity) -%}
  <!-- Displaying each item separately -->
{%- endfor -%}

However, when I try to update the properties for a specific "split" item (via the line property in the Shopify cart API), it updates all the items in that line instead of just one.

Here’s a snippet of how I'm trying to update the line item in the cart:

const item_variant_id = this.querySelector('input[name="item_variant_id"]');
      const item_variant_key = this.querySelector('input[name="item_variant_key"]');
      const line_item_index = this.querySelector('input[name="line_item_index"]');


const body = JSON.stringify({
    line: line_item_index.value,  // 1-based index of the line item in the cart
    quantity: 2,  // just for testing
    properties: {
        "FROM": nombreField.value,
        "FOR": celularField.value
    }
});