I can not modify an item in mercadolibre with php

I have this code which theoretically allows me to modify the stock of an item. He reads it, he does not give an error, but he does not modify it.

<?php 
  $ACCESS_TOKEN="APP_USR-3743017...";
  $uso=1;
  $detallesART = '{"available_quantity":99}';
  $ITEM_ID="MLA663524414";
  $url="https://api.mercadolibre.com/items/$ITEM_ID?access_token=$ACCESS_TOKEN";
  $ch = curl_init($url);
  curl_setopt ($ch, CURLOPT_GET, 1);
  curl_setopt ($ch, CURLOPT_GETFIELDS, $detallesART);
  curl_setopt($ch,CURLOPT_RETURNTRANSFER,true);
  $respuesta=curl_exec($ch);
  echo "<br>OK:".$respuesta;
  $error = curl_error($ch);
  echo "<br>Error:".$error;
  curl_close ($ch);
?>