To change the order of items that appear in the navigation menu go to the 'Pages' tab within the 'Edit site properties' area.
To change the order click "Up" or "Down" next to selected item.
You can also move items to sub-menus and back by clicking "Make sub level" or "Make top level".
To hide the page from the menu click "Hide". To restore it click "Show".
You can also add additional items for which you can specify the URL by clicking "Add a page".
Those items can also be deleted by clicking "Delete".