moving an admin submenu item to a top-level position

There is no easy to way do this, I answered this question with a code example on WPSE recently, but I cannot find it.

Basically what you have to do for sub-menu items ordering is remove them all, then add them in the order you want.

So first you have a function to remove them:

//plugin sub-menu editor as the first example
remove_submenu_page( 'plugins.php', 'plugin-editor.php' ); 
remove_submenu_page( 'next one' );  
remove_submenu_page( 'next one' );  

Then you use add_submenu_page and add them in the order you want. http://codex.wordpress.org/Function_Reference/add_submenu_page

It seems convenluted and it is, the alternative is something wacky like this, Order Admin sub-menu items?

This is for sub-menu items, for top level items, use remove_menu_page

Edit

Index.php is not submenu item , it is a top level item so you title is confusing!, to order top level items use this answer, https://wordpress.stackexchange.com/a/1568/1509