Each book, chapter and page has a title shown at the top.
If you move your mouse to the immediate right of the title a pencil will appear, click on the pencil and you will see
You can change the title of the chapter or page simply by typing. If you change the numbers at the left hand side this will affect the title which is shown but it does not change the URL for the page or the position in the guide. To do that you have to click on the link symbol at the left which will turn the gray box white so you can end it.