This template was not designed to allow that kind of change easy. It might be done but that kind of changes are beyond our support.
Just to give you a clue if you know html css and js - you would need to create js function that checks scrollTop value, and if it is bigger than menu top position, you would need to change menu's class so it's position changes to static. You might base on solution in creativity template. Some custom css might be also required to achieve complete solution.