If you ever need to change the details of the external link you can do so by editing the appropriate information on the Page Settings tab.
Simply select the external link you need to edit the details for in the Web pages list on the Web pages tab. You will then be able to edit the information relating to that external link on the Page Settings tab. For example, if the url changes simply enter the new url in the Link URL box or if you want to change how it is listed on the navigation menus edit the Link Text field.
You can also change which menus the link will appear on, and whether to include details about this external link on your Site map.
Note: Because an external link is not a real page you will not be able to edit the design, view the source or run page analysis on it and so all of these tabs have been disabled.