If an upgrade is available, it will be offered to you when viewing your booking on the KLM website while logged in to your Flying Blue account.

You cannot upgrade using miles as part of the purchase process of buying a ticket on the KLM website.

Flying Blue now operates variable pricing; you won't know exactly how much they will ask until you actually have a booking and look it up.

I would, however, advise you find a better way of getting rid of your miles - upgrade pricing is terrible.

(You can donate miles to charity before they expire)

As you see here, it is frequently as expensive, or even more expensive, to upgrade with miles than to redeem into the Business cabin outright using miles...