Category Management
You can add / remove categories on a specific Post within that Post in the editor.
New categories can be added either while editing or creating a Post or via Posts > Categories. Fill out (at a minimum) the category name to add a new category via this second route.
To globally edit an existing category, go to Posts > Categories and click on the name of the Category you would like to access. Edit any necessary fields in the window that appears and click update.
To globally remove/delete an existing category, go to Posts > Categories and hover over the name of the Category you would like to remove. Click the Delete link that appears below the category name upon hover.
Next up: Tag Management→