You cannot rename them nor remove/delete items in the menu.
Here's the list of URL parameters you can use to affect some of the display options:
There isn't an out box feature to do what you're asking for. You can build your own portal/landing page that will let you do that though. You can embed the viz with the url parameter toolbar=no, from Toby's reference link, and use the java api to build your own download options, eg.
Alternatively, you can use a third party software. Here is pretty neat example of custom portal by Interworks (I'm not affiliated). Check out the download options. They removed the button download workbook and added export to power point. The demo is part their power tools for Tableau, Portals for Tableau.