Update form Cakephp 3

I am currently working on a project using cakephp 3.

I have a form to add clients which works using this in my controller :

    public function add(){

        $clients = $this->Clients->newEntity();
        if($this->request->is('post')){
            $clients = $this->Clients->patchEntity($clients, $this->request->data);
            if($this->Clients->save($clients)){
                $this->Flash->success(__('Client has been created.'));
                return $this->redirect(['controller'=>'Clients','action'=>'index']);
            }
            $this->Flash->error(__('Client hasnt been created.'));
        }
        $this->set('clients',$clients);

    }

Then I want to have the possibility to modify one of my client. I have a table of clients and when I click on of them, I have a modify button coming (jQuery). Then I'm on my modify page. I did some test with the doc from cake but it seems I don't understand how it works and what tools should I use.

For the moment, I have this on my Controller:

public function modify($id = null){
            if(empty($id)){
                throw new NotFoundException;
            }
            $clients = $this->Clients->get($id);
            /* there should be the modify code */
            $this->set('clients', $clients);

        }

I don't really know what to use as I said... Any help pls?