Hi,
The categories can not be reset, you need to re-import the demo data. You need to remove all imported pages/posts. I don’t suggest you re-import demo data
“all” / “print” / “ux ui design” / “prototypage”
These items are the filters, yes, they are from the categories, plz refer to how to set the categories: https://seatheme.net/forums/topic/arnold-wordpress-theme-faqs/#post-15242
ps: they are only be shown on the portfolio page.
“contact” and “about”
These items are not categories, they are menu(Appearance>Menus)
Regards