You can edit product groups in the following window, available under Accounts/Fees/Edit and change group of articles:

1. You can create a new product category after pressing New (F9).
2. You can modify and change the categories by pressing Edit (F7).
3. 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.
4. The Exchange article groups within the articles opens the following window; confirm your settings with OK.