Zend certified PHP/Magento developer

How do you install an extension from GitHub?

I’m contemplating to buy this extension from Firebear studio:

https://firebearstudio.com/the-improved-import.html

In order to move products from Magento 1.9.3 to Magento 2.

They have replied the following:

In regard to Magento 1. You can use default magento export or in addition our free extension

https://github.com/firebearstudio/iimagento1

Note. This extension is no longer supported and we do not provide services to install and configure this extension.

Now, I’ve gone to that GitHub page, clicked EVERYWHERE and I can’t find any installation instructions! How do I install this? Do I just upload the 2 files via FTP? How do I start the extension? Why does it not include any installation instructions while at the same time, the company who created it and points me to it, provides NO support?