Update by your code

curl --request PATCH \
  --url https://staging2.xfaster.net/api/partner/v1/items/external/{external_id} \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "price": 590,
  "stock": 40
}
'

const options = {
  method: 'PATCH',
  headers: {Authorization: 'Bearer <token>', 'Content-Type': 'application/json'},
  body: JSON.stringify({price: 590, stock: 40})
};

fetch('https://staging2.xfaster.net/api/partner/v1/items/external/{external_id}', options)
  .then(res => res.json())
  .then(res => console.log(res))
  .catch(err => console.error(err));

import requests

url = "https://staging2.xfaster.net/api/partner/v1/items/external/{external_id}"

payload = {
    "price": 590,
    "stock": 40
}
headers = {
    "Authorization": "Bearer <token>",
    "Content-Type": "application/json"
}

response = requests.patch(url, json=payload, headers=headers)

print(response.text)

<?php

$curl = curl_init();

curl_setopt_array($curl, [
  CURLOPT_URL => "https://staging2.xfaster.net/api/partner/v1/items/external/{external_id}",
  CURLOPT_RETURNTRANSFER => true,
  CURLOPT_ENCODING => "",
  CURLOPT_MAXREDIRS => 10,
  CURLOPT_TIMEOUT => 30,
  CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
  CURLOPT_CUSTOMREQUEST => "PATCH",
  CURLOPT_POSTFIELDS => json_encode([
    'price' => 590,
    'stock' => 40
  ]),
  CURLOPT_HTTPHEADER => [
    "Authorization: Bearer <token>",
    "Content-Type: application/json"
  ],
]);

$response = curl_exec($curl);
$err = curl_error($curl);

curl_close($curl);

if ($err) {
  echo "cURL Error #:" . $err;
} else {
  echo $response;
}

{
  "id": 3062,
  "external_id": "BURG-SMASH",
  "category_id": 1201,
  "name": "סמאש בורגר",
  "price": 6000,
  "discount": {
    "type": "amount",
    "value": 500
  },
  "sale_price": 5500,
  "available": true,
  "off_until": null,
  "barcode": null,
  "unit": null,
  "by_weight": false,
  "stock": null,
  "option_groups": [
    {
      "name": "מידת עשייה",
      "type": "single",
      "min": 1,
      "max": 1,
      "options": [
        {
          "name": "מדיום",
          "price": 0
        },
        {
          "name": "וול דאן",
          "price": 0
        }
      ]
    }
  ],
  "addon_ids": [
    412,
    413
  ]
}

{
  "error": {
    "code": "not_found",
    "message": "No such order for this store.",
    "details": null
  }
}