This is a very simple template. It's meant to show developers the structure and required components of a Squarespace website. It has just two pages and limited functionality.
To make changes, first enable developer mode by going into the Settings > Advanced > Developer Mode menu. Then download the template via SFTP, or clone it with git.
You can add pages by going to the Pages menu, adding a page, and editing it's content. If you have HTML code, you can copy it into a code block. If you prefer to edit HTML using your own tools, create a .page file containing HTML, and put it in the pages folder of your template source code. Once you've added the page, go to the Pages menu and drag the new page into the main navigation.