To change the name of a page on the Menu, click Settings in the Menu element.
In the Settings window, select the page that you want to re-name. Type the new name in the label field.
Note that the original name will still be used internally for navigation but won't be shown to visitors.
Note that the menu is a template element, so when you change it on one page, the changes are automatically made on all pages.
When finished, click Save & close.