Your website runs on a CMS (Content Management System) called Concrete5. This gives you great power - to add / edit / delete pages, alter your site navigation system, edit your site content etc.
Concrete5 has detailed tutorials on how to do all this.
Please see here: https://legacy-documentation.concrete5.org/editors/in-page-editing/adding-pages
Please see here:https://legacy-documentation.concrete5.org/editors