How to edit the first page:

Click on the settings gear for the page and change the layout from Front Page to single page

Make your changes

Change the layout back to Front Page.

 

Also -  in the custom css of the site, there statements that hide two sections with a headline and the embedded video when on a large screen but show them on a small screen.

If you edit the front page, the div IDs referenced in that CSS may change.

If so, use a web inspector to get the new IDs and update the css.