Overwriting the same row with ajax

I have the following ajax and php files to enter data to my current mytable table. The below codes work together perfectly fine as they are.

Ajax file:

<script type="text/javascript">

  function insertData() {
    var stata=$("#stata").val();
    var statb=$("#statb").val();
    var statc=$("#statc").val();


        $.ajax({
            type: "POST",
            url: "update.php",
            data: {stata:stata,statb:statb,statc:statc},
            dataType: "JSON",
            success: function(data) {
             $("#message").html(data);
            $("p").addClass("alert alert-success");
            },
            error: function(err) {
            alert(err);
            }
        });

}

</script>

update.php

<?php
include('db.php');
$stata=$_POST['stata'];
$statb=$_POST['statb'];
$statc=$_POST['statc'];

$stmt = $DBcon->prepare("REPLACE INTO mytable (stata,statb,statc) VALUES(:stata, :statb,:statc)");

$stmt->bindparam(':stata', $stata);
$stmt->bindparam(':statb', $statb);
$stmt->bindparam(':statc', $statc);
if($stmt->execute())
{
  $res="Data Inserted Successfully:";
  echo json_encode($res);
}
else {
  $error="Not Inserted,Some Problem occur.";
  echo json_encode($error);
}

?>

What I'd like to do is to overwrite the same row (update or replace)in the table ( like where id=1) every time I enter the data.

But when I add "Where id=1" to the sql query, I get [object Object] error on the screen. I couldn't make it work any way I tried. How can I overcome this problem? thanks.