Navigate to this option by going to your website admin, browse to Appearance > Customize > Menus > Select a Menu.
You can place a specific menu item before or after another. Just drag the menu items and reorder them as per your preferences.
When you are done performing the reorder, you can hit publish to see the changes in action. If you just want to get an idea of how it looks, you can get a live preview as you drag the menu items. So you only have to publish once, after you like what you see.