To edit one of the links, select all of the link text of an existing link and overwrite it with your own text. Then, with the text still selected, edit the Link box on your Property inspector to reflect the page or file you wish to link to.
To insert a new link in the navigation menu, place your cursor at the end of any of the existing links and press Enter to create a new line. Then choose Insert - Hyperlink. Fill out the Hyperlink dialog to reflect the link you wish to make and click OK. The new link will appear on the new line you created.