function UpdateTable()
{
    global $connection;
    $username = $_POST['username'];
    $password = $_POST['password'];
    $id = $_POST['id'];
    $query = "UPDATE users SET "; // username is the name of the column in the database, you need the space in  between
    $query.="username = '$username' , ";
    $query.= "password = '$password' ";
    $query.= "WHERE id = $id ";
    $result = mysqli_query($connection,$query);
    if(!$result)
    {
        die("Query failed". mysqli_error($connection));
    }
}