After focusing on categories and tags in my last post, I took a look at the Categories box in my dashboard and saw there was one unused category that I wanted to remove from my All Categories list. I had to do a little research to find out how to do this.
I discovered that I just needed to select the Categories option under the Posts section in the dashboard. If you select this option, you will go to a page that allows you to see another list of your categories and to manage them. You just select the category that you want to delete and then choose the Delete option that appears under the title of the category when you move your mouse over the name of the selected category.
One thing to note, however, is that posts that were posted with the deleted category will now be posted with the Uncategorized category.
Here is a link to a WordPress Support page that will show you how to not only delete categories but also how to add categories, change the default category of your posts, and convert categories to tags.