By default all your pages in a Google site’s navigation will be sorted and arranged alphabetically. This can be problematic if you need a specific order. To have manual control over the ordering of your site’s pages, follow these simple instructions.

Go to More  -> Manage site -> Site layout and click the edit link under the sidebar Navigation. A Configure Navigation screen should appear.

Uncheck Automatically organize my navigation. You should now see a list of the pages you’ve created in the site. Use the arrows to move the pages around in the order you need. Remember to select OK when you have finished arranging your pages.

This is the same process whether you need to reorder vertical or horizontal navigation. Click edit horizontal nav content on the Site layout page located just under the Header section if your site uses horizontal navigation. Use the arrows to rearrange the order of the pages on the horizontal navigation bar.