Tableau Server includes a feature known as 'webedit' that allows you to make changes to workbooks that have been published to the server. It doesn't have the full functionality of Tableau Desktop but there is a lot you can do with it. Tableau Desktop is not actually bundled with Tableau Server, it really is its own separate product.
For Tableau Server the product key will start with "TS...." whereas for Tableau Desktop it is "TD......". More than likely someone in your organization has keys for Tableau Desktop. You can download the trial version of Tableau Desktop from our site to get started. If you then apply a valid Tableau Desktop key it will then convert from a trial installation to a regular installation.