1. Go to "appearance" > "menus".
2. Select the menu that you wish to edit from the drop-down list.
3. Then, click the "select" button.
4. Click the "most recent" tab under Pages.
5. If you can't find the pages that you wish to add, click on "view all" tab.
6. Tick on the page that you wish to add.
7. Click "add to menu".
8. Drag and drop the latest menu item to the desired place.
9. The arrangement is as below.
No. 1 = main item.
No. 2 = sub item of no. 1.
No. 3 = sub item of no. 2.
9. Click "save menu" once done.