// connect.php
<?php
  $connection = mysqli_connect('localhost', 'root', 'root', 'loginapp');
   if(!$connection) {
       die("Database connection failed");
   }
 ?>
// showDb.php
<?php include 'connect.php' ?>
<?php
  function showAllData() {
      global $connection;
      $query = "SELECT * FROM users";
      $result = mysqli_query($connection, $query);
      if(!$result) {
        echo(mysql_error());
      }

      while($row = mysqli_fetch_assoc($result)) {
        $id = $row['id'];
        echo "<option value='$id'>$id</option>";
      }

  }
 ?>
// update.php
<?php include 'showDb.php'; ?>
<?php
  if(isset($_POST['sumbit'])) {
    $username = $_POST['username'];
    $password = $_POST['password'];
    $id = $_POST['id'];

    $query = "UPDATE users SET ";
    $query .= "username = '$username', ";
    $query .= "password = '$password' ";
    $query .= "WHERE id = $id ";

    $result = mysqli_query($connection, var_dump($query));
    if($result) {
      echo "Success";
    } else {
      die(mysql_error());
    }
  }

 ?>

// some html...
<select name="id" id="">
                  <?php
                  showAllData();
                    ?>
               </select>