webman-exercise/app/controller/User.php

<?php

namespace app\controller;

use support\Request;
use app\model\User as UserModel;

class User
{
    public function index(Request $request)
    {

        $user = UserModel::first();

        if ($user) {
            $user->username = 'updated';
            $user->save();
        }
        return json($user);
    }

}