From the admin pages open the Content menu and choose Content Management.

To reorder the main navigation, click on the name of your site in the Site Map on the left. If you want to reorder subitems within a folder of your site, click on that folder in the Site Map.

From the Index tab for the selected item, click the Reorder button.

To move an item click its up ▲ or down ▼ arrow to change its position. When you are happy with the new order, click the Save changes button.