$editID = $_GET['editID'];
if(isset($_POST['edit'])) {
  $brandEdit = $_POST['brandEdit'];
  $serialEdit = $_POST['serialEdit'];

  $editInventory = "UPDATE `inventories` SET `brand` = '$brandEdit', `serial` = '$serialEdit' WHERE `inventories`.`inventory_id` = '$editID'";
  $conn->query($editInventory);
}
?>

      <div class="col-md-12">
        <form method="POST" action="inventories.php">
              <input type="submit" name="edit" value="UPDATE">
        </form>