To edit or delete categories that you have already created, you will first want to be in your blog dashboard.
From the dashboard menu, hover over "Posts". You will see the "Categories" item appear when you are hovering, as pictured in the screenshot below. Click Categories.
On that page, you can see a complete list of the categories that exists on your blog currently
If you hover over each category, you will have the option to Edit, Quick Edit, Delete, and View.
By clicking "Delete", the category will be deleted. Any posts that are filed under the deleted category will be moved to "Uncategorized".