Renaming a page is done via “Manage Pages”, accessed by clicking the “Select Page” at the top of the side menu.
Find the page you want to rename, click the “Rename” icon, enter the new name and press save. If you rename a page it will also update the URL of the page. Any existing links you've either sent out to people or those indexed on the internet to that page will no longer work so try to avoid frequent renaming unless necessary.
The new name will also automatically be updated in your sites navigation.