Today easy and short tip, but you could wander around for hours searching this:
When you have bought new licences you can easily add key to the subscription.
Go to the Volume Licencing Center at:
(If you have not logged in earlier here you need to register with e-mail account provided with licence order)
Get (copy) your subscription key from the license.
Next go to this link:
and paste the key.
If you have used trial licences you'll have to change your user's licences from trials to the new one.