You don't need to delete your existing product or add a new product. Just Edit the product you already have. You'll find the Upload Product File button right there on the first screen of product settings when you edit an existing product.
So, by "landing page" you were actually talking about a shopping cart that you'd added items to, testing as if you were a buyer building an order? Perhaps you had added the original product to your cart order, then you deleted that product in Admin and created a replacement product, and then when you added that new item to your testing cart order, the cart couldn't tell right away if the original product had just been deleted, so it still showed up in that particular test order.
If you're only selling the one product and not using Tax or Shipping calculations, perhaps it would be easiest if you just used a Buy Now button for instant checkout, rather than using cart buttons?
