By default, the items in the left-hand navigation menus appear in alphabetical order (with hidden pages appearing in alphabetical order beneath the non-hidden ones). To change the order of the visible menu items, click the Settings tab and then click Reorder menu from the toolbar.
To move a menu item to a different location on the list, click on the four-pointed arrow beside the item and drag it to its new location on the list, dropping it in place by when you release the mouse button. Repeat until you have the menu items in the order you want, then press the Save Configuration button.
Note that when create a new sub-page on a page where you've reordered the menu, the new menu item will appear at the top of the list. You'll have to use Reorder menu again to move the new item to another location on the list.
Note that you can always revert to the default alphabetical order by click the Remove custom ordering and save button. You can also choose to order the menu items in reverse alphabetical order by clicking the Reorder Z to A and save button.