To renew or extend your Microsoft 365 Family or Personal subscription with a product key, follow this guide:
Go to office.com/myaccount and sign in with your Microsoft Account
This will require your email, phone number or Skype account plus your password
For further information, you can also go to the following website link: Microsoft renew with a product key link
NOTE: The subscription is added to the expiry date, not from the date you renew it on
NOTE: Also, you can downgrade from a Home to Personal subscription or vice versa