Delete labels and categories
You can delete individual labels or an entire category and all the labels that it contains. If you delete all the labels within a category, then the category is also deleted automatically.
When you delete a label from the Configure Labels dialog, it is not removed from documents. You can, however, simultaneously delete labels and remove them from the Assign Labels dialog.
If you delete labels without removing them from documents, then the next time you select these documents when you are assigning labels, you'll see a category called "Undefined Labels" at the bottom of the Assign Labels dialog. You can remove the labels from the documents at this time.