$id= $_GET['id'];
$sql = "UPDATE tabelnaam SET kolomnaam='value' WHERE id=$id";

<?php
include "connect.php"; 

$id= $_GET['id_order'];
$aantal= $_POST['aantal'];

//Selecteer je tabelnaam"; 
$sql = "UPDATE Order_Bo SET aantal='$aantal' WHERE id_order='$id'"; 

       if (!mysqli_query($conn, $sql)) {
       die('An error occurred when submitting your review.');
       }

       include('order.php');
?>