function updateTable() {
    global $connection;

    $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, $query);

    if (!$result) {
        die("Query Failed" . mysqli_error($connection));
    }

}

// HTML FILE
<?php
if(isset($_POST['submit'])){
    updateTable();
}

?>
  
  <form action="update.php" method="post">
    <select name="id" id="">

        <?php
        showAllData();

        ?>

    </select>

    <input type="text" name="username">
    <input type="password" name="password">
    <button type="submit" name="submit">Update</button>
</form>