<input type="number" onchange="changeQuantity(this.value, '{{$cartProduct->id}}')" id="quantity" class="form-control text-center" value="{{$cartProduct->quantity}}">


 <script>
        $.ajaxSetup({
            headers: {
                'X-CSRF-TOKEN': $('meta[name="csrf-token"]').attr('content')
            }
        });

        function changeQuantity(quantity, productId) {
            $.ajax
            ({
                type: "post",
                url: '{{ route('cart.updateproduct') }}',
                data: { quantity: quantity, productId: productId }
            })
        };
    </script>

Route::post('/shoppingcart/updateproduct',  'ShoppingCartController@updateProduct')->name('cart.updateproduct');

    /***
     * Update existing product quantity
     * @param Request $request
     */
    public function updateProduct(request $request){

        $user = Auth::user();

        if(is_numeric($request->quantity))
        {
            DB::table('cart_product')->where([
                ['productId', $request['productId']],
                ['cartId', $user->cartId]
            ])->update(array('quantity' => $request['quantity']));
        }
    }