How to download the game
After preordering Overgrowth and linking a forum account, access to the Secret Preorder Forum becomes available. Each week, the alpha build is posted as a stickied topic with a title of the form: "Unofficial Overgrowth a###, weekly build".
There are multiple download options available - a direct download, torrent download, and a "differences" download, which can be used to update a previously installed alpha.
Additionally, once at least one full alpha installation has been performed, the community-made SUMLauncher may be used for updates.
Overgrowth Tutorial - Getting Overgrowth (Mac & Windows) - YouTube video by Silverfish