I purchased Microsoft 365 and only have a serial number - how do I activate it

If you have a 25 digit product key, go to https://setup.office.com , sign in with a Microsoft account and then enter the key.

To activate Microsoft 365 with a product key (serial number), the key must first be redeemed and associated with a Microsoft account. After that, installation and activation are done by signing in, not by re‑entering the key on the device.

Follow these steps:

1. Redeem the product key
   • If this is a new, never‑used key, go to either:
     • https://www.microsoft365.com/setup, or
     • https://setup.office.com
   • Sign in with an existing Microsoft account, or create a new one when prompted.
   • Enter the product key and follow the on‑screen prompts to add the subscription to that account. This is a one‑time process.
2. Install Microsoft 365
   • After redeeming, sign in to the same Microsoft account on the setup site and start the installation from there.
3. Activate on the PC
   • Open any Microsoft 365 app (for example, Word).
   • When prompted to activate, choose to sign in.
   • Sign in with the same Microsoft account used to redeem the product key. The product key itself is not normally entered again; activation is tied to the account.

If Microsoft 365 prompts for a product key again later, sign in with the correct Microsoft account instead of entering the key. Make sure the account used is the one that was used to buy or redeem Microsoft 365.

References:

• Why do I need an account to sign in to Microsoft 365 or Office?
• Using product keys with Microsoft 365
• Activate Office for Windows
• Try or buy Microsoft 365