In Ubuntu, we run System/Administration/Synaptic Package Manager to install
programs. There are other options, "software center" or the command line, but
Synaptic is a good choice.

Then you "search" (not quick search) for Stellarium. Click the box on that line
and select "Mark for Installation." Then click the Apply button.

Finding a zip on the Internet is a good approach for Windows, but it's a
terrible approach for Linux.