If you mistakenly added a category, you can remove it in editor mode. If you want to delete a category, you can delete the page by clicking on "Categories" and going to the list then removing/deleting the one that is incorrect.
You go to visual/source editor, go to the right side of the page, you'll see a scroll box where all the categories are. Hover your mouse over the category you want not, and a little trashcan symbol should appear. Then press Save. Voila.