Here are the steps to do so:

1. Log in to HubSpot and go to Marketing > Website > Website Pages from the main top menu.
2. From the top right corner button, click Create > Website page.
3. Then, select the Reformer theme.
4. On next page, select any of the available templates.
5. Give this new page a name, like Home.
6. Click the Create page button.

After this, you will have a page with the demo content which you can edit as you wish. You can edit or/and remove any of the existing modules and/or drag/drop any other modules.

Please note: To set the home page as the default page when someone reachs the website address, you need to edit the settings of your home page and remove the content slug (which should currently contain home or /home) so that just the root subdomain is shown. Then update.