Edit product types
You can edit product groups in the following window, available under Accounts/Fees/Edit and change group of articles:
You can create a new product category after pressing New (F9).
You can modify and change the categories by pressing Edit
You should Delete
a group only when no articles are part of it. When you delete a product category that is still used, the category's abbreviation is saved in the article's description.
The Exchange article groups with the articles opens the following window; confirm your settings with OK.