<form action="Actualizar Estado.php" method="POST">
                <div class="row">

                    <div class="col-12 col-lg-8">
                        <div style="padding-left:80px;">
                        <input type="text" name="idreserva" class="form-control" placeholder="  ¿Id de Reserva?" >
                    </div>
                       <div style="padding-left:80px">
                    <select  name="estado" class="form-control" >
                            <option value="nada" >¿Estado del Pedido?</option>
                            <option value="completo">Completado</option>
                            <option value="anulado">Anulado</option>
                        </select>
                    <p></p>
                        </div>
                    </div>
                    <div class="col-12">
                        <div style="padding-left:140px" >
                        <button onclick="cambiarEstado()" name="submit" type="submit" class="btn btn-primary p-3 px-xl-4 py-xl-3"><span></span> Cambiar Estado</button>
 </form> <!-- no olvides cerrar el formulario --->

 <script>

        cambiarEstado() {
             var form = document.querySelector('form');
             var data = new FormData(form);

             var params = typeof data == 'string' ? data : Object.keys(data).map(
        function(k){ return encodeURIComponent(k) + '=' + encodeURIComponent(data[k]) }
    ).join('&');

             // example request
             getAjax('scriptPHPParaUpdate.php', params);
        }

        function getAjax(url, params) {
            var xhr = window.XMLHttpRequest ? new XMLHttpRequest() : new               ActiveXObject('Microsoft.XMLHTTP');
            xhr.open('POST', url);
            xhr.onreadystatechange = function() {
                if (xhr.readyState>3 && xhr.status==200)
                    success(xhr.responseText);
                else
                    error(xhr.responseText);
            };
          xhr.setRequestHeader('X-Requested-With', 'XMLHttpRequest');
          xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
          xhr.send(params);
          return xhr;
   }

   error(response) {
        console.log(response);
   }

   function success(response) {
        console.log(response);
   }

 </script>