As explained in the “Who owns my data”, Stastic lets you edit content stored on Github servers, in a project folder or “repository”.
When you open Stastic editor as explained in the section “How to access Stastic admin?”, you have access to a side bar or a menu on mobile:
There you can see the “Github source” link which leads to where your website’s files are stored. You can check it out, edit files on Github, and come back to Stastic.
You might also be interested in the section “What is Github pages?”.