How to change the wordpress navigation bar UL to OL in Twenty_Twelve similer theme

by

Within the header, modify the wp_nav_menu code to include:

'items_wrap' => '<ol id="%1$s" class="%2$s">%3$s</ol>',

Full Menu Code

<?php wp_nav_menu( 
  array( 
    'theme_location' => 'primary', 
    'menu_class' => 'nav-menu', 
    'items_wrap' => '<ol id="%1$s" class="%2$s">%3$s</ol>' 
  ) 
); ?>