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 - Oct 15, 2018
Tableau Server Upgrades
If you are upgrading a Windows based Tableau Server with SSL enabled to 2018.2, please upgrade to latest maintenance release 2018.2.2. to avoid known issue ID #808909. Tableau Server 2018.2 comes with a new web-based server configuration utility called Tableau Services Manager (TSM). Upgrading to this version requires special steps. Please see this article for instructions on what has changed to ensure your upgrade goes smoothly.