methods
/////////////////////////////////////////////////////////////////////////////////
public static function find_by_id($uid=0){
        // return an object populated based on user id
        global $database;
        $user = new User();

        $sql = "SELECT * FROM users ";
        $sql .= "WHERE  id=${uid} ";
        $sql .= "LIMIT 1";

        $result = $database->query($sql);

        if($database->num_rows($result)){
            $row = $database->fetch_assoc($result);
            $user->username = $row['username'];
            $user->password = $row['password'];
            $user->id = $uid;
        }

        return $user;

}

public function update(){

        global $database;

        $sql = "UPDATE users ";
        $sql .= "SET username = '" . $user->username . "' password = '" .  $user->password . "' ";
        $sql .= "WHERE id ='" . $user->id;

        $result = $database->query($sql);

        return $result;

}

///////////////////////////////////////////////////////////////////////////

update.php

<?php
require_once('database.php');
require_once('user.php');

$user = User::find_by_id($_GET['id']);

if(isset($_POST['submit'])){
    $user->id = $_GET['id'];
    $user->update();

}

<form action="update.php?id=<?php echo $user->id; ?>" method="post">
    <label for="username">Username</label>
    <input type="text" name="username" value="<?php echo $user->username; ?>" />
    <label for="password">Password</label>
    <input type="password" name="password" value="<?php echo $user->password; ?>" />
    <input type="submit" value="Update" name="submit">
</form>