Update a Line Item

const options = {
  method: 'PATCH',
  headers: {Authorization: 'Bearer <token>', 'Content-Type': 'application/json'},
  body: JSON.stringify({line_item: {variant_id: '1', quantity: 2}})
};

fetch('http://{defaultHost}/api/v2/platform/line_items/{id}', options)
  .then(res => res.json())
  .then(res => console.log(res))
  .catch(err => console.error(err));

{
  "data": {
    "id": "30",
    "type": "line_item",
    "attributes": {
      "quantity": 4,
      "price": "10.0",
      "created_at": "2022-11-08T19:34:21.508Z",
      "updated_at": "2022-11-08T19:34:21.772Z",
      "currency": "USD",
      "cost_price": "17.0",
      "adjustment_total": "0.0",
      "additional_tax_total": "0.0",
      "promo_total": "0.0",
      "included_tax_total": "0.0",
      "pre_tax_amount": "40.0",
      "taxable_adjustment_total": "0.0",
      "non_taxable_adjustment_total": "0.0",
      "public_metadata": {},
      "private_metadata": {},
      "display_discounted_amount": "$40.00",
      "display_amount": "$40.00",
      "display_final_amount": "$40.00",
      "display_subtotal": "$40.00",
      "display_pre_tax_amount": "$40.00",
      "display_price": "$10.00",
      "display_adjustment_total": "$0.00",
      "display_additional_tax_total": "$0.00",
      "display_promo_total": "$0.00",
      "display_total": "$40.00",
      "display_included_tax_total": "$0.00"
    },
    "relationships": {
      "order": {
        "data": {
          "id": "35",
          "type": "order"
        }
      },
      "tax_category": {
        "data": {
          "id": "75",
          "type": "tax_category"
        }
      },
      "variant": {
        "data": {
          "id": "192",
          "type": "variant"
        }
      },
      "adjustments": {
        "data": []
      },
      "inventory_units": {
        "data": []
      },
      "digital_links": {
        "data": []
      }
    }
  }
}