You recognize on the email you receive if it is an upgrade.
The email looks like this:

Upgrade (activationkey) for 3 more licenses	Shows you for how many licenses the upgrade is.
Import CRMADDON COUNTER Professional IT.NET	Which addon and version is it for?
CGFSX-8Fd95-u23cW-BtW8P-WPC5M-36A59	This shows the new license key

 

If you want to add additional licenses or to upgrade your addon, go to 'Show License Info'.



The following window appears.
There you need to click on 'Activation Key' and enter the activation key in the next window.
 


Afterwards you can see how many licenses you have and how many are still available.