Update a Stock Item

const options = {
  method: 'PATCH',
  headers: {Authorization: 'Bearer <token>', 'Content-Type': 'application/json'},
  body: JSON.stringify({
    stock_item: {
      variant_id: '2',
      stock_location_id: '2',
      count_on_hand: 200,
      backorderable: true
    }
  })
};

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

{
  "data": {
    "id": "291",
    "type": "stock_item",
    "attributes": {
      "count_on_hand": 200,
      "created_at": "2022-11-08T19:35:29.347Z",
      "updated_at": "2022-11-08T19:35:29.598Z",
      "backorderable": true,
      "deleted_at": null,
      "public_metadata": {},
      "private_metadata": {},
      "is_available": true
    },
    "relationships": {
      "stock_location": {
        "data": {
          "id": "166",
          "type": "stock_location"
        }
      },
      "variant": {
        "data": {
          "id": "279",
          "type": "variant"
        }
      }
    }
  }
}