In the administration menu go to Site Building -> Menus
Click on Main
Click the cross of the menu you would like to move and drag it to the
location you would like to move it to. You can drag it left and right
to either make it a sub menu of another menu or remove it from a submenu
of another menu.
Click Save Configuration.
If you move a menu item left or right you will need to make additional
adjustments to make sure the url path is correct.
After you save the menu, click on the name of the menu item you have moved.
Click on the Edit Tab
Go to the bottom and click Save
This will reset the url so it reflects the correct menu path.