Howto - Change error pages

You can set any drupal page as a custom 404 (not found)  and 403 (access denied) error pages. To do this navigate to "Administer -> Site Configuration -> Error reporting" and set the page you want to be displayed.

If you only want to change the content of an existing custom error page you have to search it under "Administer -> Content" or simply go to the URL of the error page (get it from Administer -> Site Configuration -> Error reporting) and click on edit.