Extract the .zip and run the executable! But really, just play it from its page on itch.io.
To make the game in Unity, you'll require some plugins that are not included due to their licenses, but all the source code and models are included.
Cameron Oltmann - Codemancer Supreme
Terra Groenewold - All gun models, title text, 3D modeling tutor
Anthony Free - Musician and Sound Designer Extraordinaire + Team Logo (Listen to his background music on SoundCloud!)
Mike LeSauvage - Ship models, Title card opening sequence, Logistics Wrangler