You can reorder where your different Page links show up in the Navigation Menu from the Menus section. The Navigation Menu is the menu of page links that appears at the top of every page on your site - similar to this:
You will use the Menus section if you want to re-order the items in your Navigation Menu. From your dashboard, click on the "Appearance" button. You will be directed to the Theme section panel, with additional options appearing on the left under the "Appearance" link. Click on the "Menus" link. You'll be brought to the Menu Editor, which will look similar to this:
From the Menu Editor you'll see a list of Pages that you have created and are available on the left. Along the right, you will see the order of your page links as it relates to the Navigation Bar.
To add a new page to the navigation, simply check the box of the page you wish to add, and then click the "Add to Menu" button. It will appear on the right in the bottom position. The page title at the top appears furthest to the left on your website and the page title at the bottom appears furthest to the right.
To reorder your pages, simply drag and drop them into the order you prefer. You can also have pages appear underneath pages in the navigation by dragging them under the title you wish them to appear under, and moving them slightly to the right. For example, in the animated image "News" and "Updates" is moved to appear below "About" by having it slightly staggered to the right:
On your website, this menu structure would look like the example below when a user clicks on "About":
Whenever you are done editing the order of your menu, be sure to click the "Save Menu" button on the bottom, otherwise your changes will not be reflected on your website.
The last option you are presented with in the Menus section is the ability to automatically add pages to your navigation as you create them. By checking this box, anytime you create a new page on your site, the title of that page will automatically appear in your site's navigation menu. If you do not check this box, your navigation menu will stay the same until you go in and add pages to it.