Before deleting the category, make sure that all the items under it are deleted first. You can only delete a category when it is empty. Follow the simple steps to delete categories.
1. Login to ManageApp. Go to Category (group)
2. Click on edit group at the top right corner
3. Choose the group which you want to delete.
4. Click on the delete icon at the top right. Your category will be deleted.
If you don't know how to delete the item then have a look at the below article.