This would be done entirely with CSS -- the concrete5 autonav block just outputs an unordered list (<ul> and <li> tags) for the menu -- it's up to you or whoever writes the theme you're using to make that list look the way you want.
So you'll want to google around for how to create "drop-up" lists with CSS (has nothing to do with Concrete5 specifically -- just make sure whatever tutorial or reference you use also builds off of an unordered list [<ul> and <li> tags]). Then whatever CSS you use to make that happen, put it into your theme's stylesheet.