There is already a way to do that when you edit pages in wordpress. Take a look at this image.In the bottom right hand corner you'll see a box that says "order" with a zero in it. You can change that number to whatever you want and the pages will sort by that number ascending order.
You should also take a look at the files in your theme and locate the wp_list_pages function that is used to render the menu you're looking at. That function takes a 'sort_column' argument that can override your menu order. Removing / modifying that argument will help you.