Cara Mengupdate Nilai pada Session menggunakan form input

Selamat sore, aku ada pertanyaan nih kak, gimana ya cara update nilai pada session? saya sudah berhasil membuat add to cart, dan menampilkan cart, sekarang saya mau belajar bagaimana cara update nilai pada session melalui form input, tampilan update cart nya begini kak : Rencananya saya mau buat shopping cart, mohon bantuannya kak.. Terimakasih :)

App\Cart.php

 <?php
namespace App;

class Cart
{
	public $items = null;
	public $totalQty = 0;
	public $totalHarga = 0;
	public function __construct($oldCart)
	{
		if ($oldCart) {
			$this->items = $oldCart->items;
			$this->totalQty = $oldCart->totalQty;
			$this->totalHarga = $oldCart->totalHarga;
		}
	}
	public function add($item, $id)
	{
		$storedItem = ['qty' => 0, 'harga' => $item->harga, 'item' => $item];
		if ($this->items) {
			if (array_key_exists($id, $this->items)) {
				$storedItem = $this->items[$id];
			}
		}
		$storedItem['qty']++;
		$storedItem['harga'] = $item->harga * $storedItem['qty'];
		$this->items[$id] = $storedItem;
		$this->totalQty += $storedItem['qty'];
		$this->totalHarga += $item->harga;
	}

}

ProductsController.php

 public function addToCart(Request $request, $id)
{
	$product = Products::find($id);
	$oldCart = Session::has('cart') ? Session::get('cart') : null;
	$cart = new Cart($oldCart);
	$cart->add($product, $product->id);

	$request->session()->put('cart', $cart);
	return redirect()->to('/');
}

public function Cart()
{
	$oldCart = Session::get('cart');
	$cart = new Cart($oldCart);
	return view('shop.cart')->with(['products' => $cart->items,'totalHarga' => $cart->totalHarga ]);
}

public function updateCart(Request $request, $id)
{
	//
}

cart.blade.php

 @foreach($products as $data)
<tr>
	<td>{{$data['item']['nama_product'] }}</td>
	<td><img src="{{ asset('images/'.$data['item']['pict']) }}" width="100"></td>
	<td>{{$data['item']['harga'] }}</td>
	<td>
		<form method="post" action="{{ route('product.updatecart', $data['item']['id']) }}">
			{{csrf_field()}}
			<div class="form-group" style="width:75px">
			<input class="form-control" type="number" value="{{ $data['qty'] }}">
		</div>
		<div class="form-group">
			<button class="btn btn-success">Update</button>
		</div>
		</form>
	</td>
	<td>
		<a href="#" onclick="return confirm('beneran?')"><button class="btn btn-danger">Hapus</button></a>
	</td>
</tr>
@endforeach

web.php

 Route::get('/cart','ProductsController@Cart')->name('product.cart');
Route::get('/add-to-cart/{id}','ProductsController@addToCart')->name('product.addtocart');
Route::post('/update-cart/{id}','ProductsController@updateCart')->name('product.updatecart');