To discard a category with multiple items in it, you have to first remove all items in that particular category.
All these changes can be made in Menu > menu name > Products.
1. Select an item (red means it is selected) and click the gear button.
2. Click Delete to erase that particular item from the list.
3. A prompt will appear, asking you to confirm the deletion of the item. Click Yes, delete it.
4. Repeat step 1 to step 3 for the rest of the items in the category.
5. Once there is no more item left in the category, click the gear button beside the category name.
6. Click Delete to erase that particular category from the list.