// First - wipe the order's existing products
$order->products()->sync([]);
or
$order->products()->delete();

// Now add some more products:
$order->products()->attach(1, ['quantity' => 50]);
$order->products()->attach(2, ['quantity' => 150]);

dd($order->products); // 0 items
$order->load('products');
dd($order->products); // 0 items


DB Schema:
table: order_product
Fields: id, product_id, order_id, quantity

the 2 products ARE in the table.