On the Edit Pages page, for any page you want to reorder, point at its name and click Quick Edit The field labelled Order is the one you are after. The field defaults to zero. When more than one page has the same order number they sub-sort by published date. Change Order to the order you want the pages displaying in.
Hint – using 5 – 10 – 15 is planning ahead, in case you later want to reorder the order.
And just to follow-up in the event that you’ve followed stvwlf’s advice and still aren’t seeing change…
There are various arguments for wp_list_pages. You’ll want to make sure one of them is for listing pages by page order.
(this is theme specific, so you’ll want to find that call in your current theme files. typically header.php)
