To remove content, you always start with the actual articles. That's 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 three drop-down boxes: Title, Ascending, and 20. The number 20 indicates the amount of articles shown in this article listing screen. Click on the 20 button and change the value to All.
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 Title heading). This way all of the items in the column are selected:
Click on the Trash button in the toolbar. Joomla shows a message confirming that the articles have been trashed.
When articles are in the trash, they're still in Joomla's database should you wish to restore them. However, you won't be needing these articles again. To permanently delete them, in the Select...