Google Sites orders the pages in your navigation automatically by default. Use this tutorial to learn how to manually reorder your navigation items.
Click on More > Manage Site
On the left sidebar, click on Site Layout
Click on Edit on the navigation box
Uncheck 'Automatically organize my navigation'
Click on Add Page, and you will see a list of the pages you have on your site
Click on the page that you want to add to your navigation, then click OK. You can't choose multiple pages at once, so you have to add the pages individually
Once you have added your pages, you can continue to reorder them by selecting a page and click on the up/down/left/right arrow buttons to arrange your pages
