Dropdown with Ajax

Question

Hello, I am doing a Dropdown with the database and I would like to use ajax to update a table, I confess that I have a difficulty in JS, I would like a help and/ or a north for this. Grateful

<!-- Profile Image -->
<div class="box box-primary">
  <div class="box-body box-profile">
    <img class="profile-user-img img-responsive img-circle" src="../dist/img/profile/'. $id .'.png" alt="User profile picture">

    <h3 class="profile-username text-center">'. $nome .'</h3>

    <p class="text-muted text-center">Ramal: '. $id .'</p>

    <ul class="list-group list-group-unbordered">
      <li class="list-group-item">
        <b>Nivel de acesso: </b> <a class="pull-right">'. $nvAcess .'</a>
      </li>
    </ul>
    <a class="pull-right image" href="#">
      <i class="fa fa-edit">Editar</i>
    </a>
  </div>
  <!-- /.box-body -->
</div>
<!-- /.box -->


<label>Users</label> </br>
<select class="form-control select2" style="width: 100%;" id="user">
  <?php                 $conn=mysqli_connect("192.168.0.2","root","Kt6v75dX","spa") or die("ERRO AO CONECTAR");
$query = "SELECT id,nome,nivel_de_acesso,senha FROM usuarios";

$query = mysqli_query($conn,$query);

$reg = mysqli_fetch_array($query);
while($reg == 0){

}

while($reg != 0){
   $id = $reg['id'];
   $nome = $reg['nome'];
   $nvAcess = $reg['nivel_de_acesso'];
   $pass = $reg['senha'];

   echo'<option value="'.$id.'">'. $nome .'</option>';

   $reg = mysqli_fetch_array($query);
}
?>
</select>