How do I upgrade from Windows 8.1 to 10 using my key?

Solution 1:

You do it exactly as the ZDNet 'Here's how you can still get a free Windows 10 upgrade' article suggests.

Go to the Download Windows 10 webpage from the computer you wish to upgrade and click the 'Download tool now' button. After the download completes, run the Media Creation Tool & select 'Upgrade this PC'.
Don't run a 'clean' install or you will lose any potential upgrade entitlement you may have.

Once upgraded, you can quickly discover whether you got it free or need your new key. It will not be shy in telling you if you need a key & present you with the opportunity to add it [but it won't lock you out, Win10 works without a key, it just nags you a lot]

Anecdotally, I recently tried this method on an old Win7 machine, really just in case it might work… and it did.
MS considers me fully licensed & genuine.

If you didn't get a free upgrade, Windows will present this dialog… often

[I don't license disposable virtual machines I use for testing.]