Redeeming
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1
Sign in to the right Microsoft account
The licence attaches permanently to whichever account you redeem it on, so pick the one you will keep using.
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2
Redeem on Microsoft's own page
Use Microsoft's setup or redeem page and paste the key from your order.
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3
Install and sign in
Download Office from your Microsoft account and sign in with the same account inside any Office app.
These are one-time licences and are different from Microsoft 365, which is a subscription. Check which one the listing is selling.