Just as the level of identation determines at what level a page is placed in your site's main menu so does the order listed. Which page is listed first on main menu and each sub-menu is based on the top-to-bottom order the page is listed on the Pages administration page.
Both the identation level and order listed can be changed (Figure 1) using the Move Page arrows. Click on the page that you would like to move* and then use the right/left arrows to change the identation and use the up/down arrows to change the order listed. The main menu will be immediately updated.
* - when a page is moved in order listed all child and sub-child pages are moved along with it.