How to update quantity in the session

hello , i built this class to store items in the cart , and i have some problem with update the quantity of the item, any help.

this Cart class

<?php

namespace App;

class Cart
{
    private $cart;

    public function __construct()
    {
        if ($this->get() === null)
            $this->set($this->empty());

        $this->cart = $this->get();
    }

    public function add(Product $product): void
    {
        $item = [
            'id' => $product->id,
            'name' => $product->name,
            'price' => $product->price,
            'quantity' => 1,
        ];

        if (!$this->exists($item)) 
        {
            array_push($this->cart, $item);
        }

        $this->set($this->cart);
    }

    public function remove(int $productId): void
    {
        array_splice($this->cart, array_search($productId, array_column($this->cart, 'id')), 1);

        $this->set($this->cart);
    }

    public function exists($product)
    {
        foreach ($this->cart as $item) {
            if ($item['id'] == $product['id']) {
                return true;
            }
        }

        return false;
    }

    public function clear(): void
    {
        $this->set($this->empty());
    }

    public function empty(): array
    {
        return [];
    }

    public function get(): ?array
    {
        return request()->session()->get('cart');
    }

    private function set($cart): void
    {
        request()->session()->put('cart', $cart);
    }
}