To edit your help center categories, navigate to the articles listing page of your help center dashboard.

There, in the toolbar at the top you will see a field named "Select a category". Clicking on it will open up a sidebar where you can see all your existing categories and modify them as you wish.

Clicking elsewhere on the page will simply close the categories tree.

You can easily create a new category by clicking on the plus icon in the Categories sidebar or right next to the "Select a category" field.