To re-arrange the menus on your main navigation, on edit mode, access the navigation pane by clicking on the arrow on the top left of your website.
Then click on Pages.
On the Page section, you will see the arrangement of your menu on the left.
They are arranged by having the first menu on top and the left side on the main navigation.
To re-arrange the menus, simply hold down the menu you wanted to move and drag it to its target position.
When you're done, the menu will be changed and saved automatically.