Usually Tableau Comes with 14 days Trial , if you want to extend tableau trail then you need to download and install different versions of Tableau.

Reach out to Tableau release website (https://www.tableau.com/support/releases)  download all version whatever available and install one by one up to higher version

Example: Install first Tableau 8.0 Version and use if for 15 days , after that uninstall tableau 8.0 and install tableau 8.1 and use it for 15 days like this you can extend trail and work on the all features and releases