To activate Tableau Desktop select Help > Manage Product Keys and then click Activate.
If you have Tableau Server only, see Activating Tableau Server.
Support Alerts - Jan 10, 2019
Tableau Server Upgrades
Windows versions of Tableau Server starting with 2018.2 come with a new web-based server configuration utility called Tableau Services Manager (TSM). Upgrading from 2018.1 or earlier to a newer version will require special steps in order to upgrade successfully. Please see this article for instructions on what has changed to ensure your upgrade experience goes well. The latest releases of Tableau Server 2018.2 and Tableau Server 2018.3 can be found on the Tableau Server release page.