Want to modify sub-menu in Magazine Basic

Hi There.
I was hoping someone could help me with a problem I have with the menu in the Magazine Basic (free version).
http://wordpress.org/extend/themes/magazine-basic
I would like the sub-nav(second nav) to only show child-pages that related to the main nav page.

In other words when you click say”about” in top menu, only “about” child pages (or categories) show up in second menu.
I am not really good with php or wp so need novice description of what to do.
Really hoping someone out there can help me with some code.(and where to put it.
Please help

[ Please do not bump, it’s not permitted here. ]