Edit in Main.elm.
Run elm reactor (in your nix shell) to get feedback from your editing, if you open an elm file (e.g. Main.elm), you will get its error feedback, if you open a html file (e.g. index.html), it will be rendered.
Compile with:
elm make src/Main.elm --output elm.js
You can then view by opening index.html in your browser.
Make sure this and elm.js are pushed to update the website.