If you need to change the category of a post, navigate to "Posts". You can change the category within the post editor (from the "Categories" box), but you can also change categories quickly using the "Quick Edit" option that appears when you hover over your desired...

You can add categories on the fly from within your post editor, using the "+ Add New Category" link in the "Categories" box. However, to edit (including change parent setup) or delete categories, navigate to "Posts>Categories". From here, you can also add new...