config-routes.php

<?php
Router::scope('/', function ($routes) {
	$routes->extensions('json');
/**
 * Here, we are connecting '/' (base path) to a controller called 'Pages',
 * its action called 'display', and we pass a param to select the view file
 * to use (in this case, src/Template/Pages/home.ctp)...
 */
	$routes->connect('/', ['controller' => 'Pages', 'action' => 'display', 'home']);

/**
 * ...and connect the rest of 'Pages' controller's URLs.
 */
	$routes->connect('/pages/*', ['controller' => 'Pages', 'action' => 'display']);

/**
 * Connect a route for the index action of any controller.
 * And a more general catch all route for any action.
 *
 * The `fallbacks` method is a shortcut for
 *    `$routes->connect('/:controller', ['action' => 'index'], ['routeClass' => 'InflectedRoute']);`
 *    `$routes->connect('/:controller/:action/*', [], ['routeClass' => 'InflectedRoute']);`
 *
 * You can remove these routes once you've connected the
 * routes you want in your application.
 */
	$routes->fallbacks();
});

curl request

curl -XPATCH -H 'Content-Type: application/json' -H 'Accept: application/json' cake3.localhost/tags/edit/2 -d '{"title": "some new title"}'

TagsController.php

<?php
namespace App\Controller;

use App\Controller\AppController;

/**
 * Tags Controller
 *
 * @property App\Model\Table\TagsTable $Tags
 */
class TagsController extends AppController {

	public $components = ['Flash', 'RequestHandler'];
	
		public function edit($id = null) {
		$tag = $this->Tags->get($id, [
			'contain' => ['Bookmarks']
		]);
		if ($this->request->is(['patch', 'post', 'put'])) {
			$tag = $this->Tags->patchEntity($tag, $this->request->data);
			if ($this->Tags->save($tag)) {
				$this->Flash->success('The tag has been saved.');
				// return $this->redirect(['action' => 'index']);
			} else {
				$this->Flash->error('The tag could not be saved. Please, try again.');
			}
		}
		$bookmarks = $this->Tags->Bookmarks->find('list');
		$this->set(compact('tag', 'bookmarks'));
		$this->set('_serialize', ['tag']);
	}
}