Deactivating 2019 and activating 2021 (offline)

I have finally upgraded my licence from 2019 to 2021 (which I purchased 2-3 years ago), with a view to upgrading to 2024 next year. However, it looks like I should have deactivated my 2019 licence first, so I can now activate my 2021 licence on my desktop. I have never been able to (de)activate online, so always have to do it offline. How can I deactivate 2019 and activate my 2021 licence please?

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