To remove content, you always start with the actual articles. This is because you cannot remove containers (categories) as long as they're not empty.
Navigate to Content | Article Manager.
Above the article listing, on the right-hand side, you'll notice two drop-down boxes: Title ascending and 20. The number 20 indicates the number of articles listed on this page. Click on the 20 button and change the value to All, as shown in the following screenshot:
Now, the full list of example articles is displayed on the page.
Select the checkbox at the top of the list (just to the left-hand side of the Status heading). This way all of the items in the column are selected, as shown in the following screenshot:
Click on the Trash button in the toolbar. Joomla shows a message that confirms that the articles have been trashed.
When articles are in Trash, they're still in Joomla's database should you wish to restore them. If this were your own articles, you...