Hey @JJeong just FYI most people here aren’t HubSpot employees so won’t have access to the CMS pages you’ve linked.
Removing a module (such as a header) does depend on how the theme has been built. More than likely, based no what you’ve said it is a coded theme and not a drag and drop template.
In order to do this, you’ll need to access your theme within your design manager and remove the header cal within the HTML/Hubl in your theme.
You’ll need to familiarise yourself with Hubl/HTML a small bit, there’s a great article on adding modules to your theme within the docs here.
You basically just need to do the reverse of that! Find your header module within the code and then remove and publish.