It should be a vertical dropdown already…..unless you’re talking about the Mobile Responsive code? If so, then take a look around line 3275 in your css for something like this –
.menu li {
float: none;
display: inline-block;
}
You could change it to this –
.menu li {
clear: both;
float: left;
display: inline-block;
}
….and it will look like this – http://i.imgur.com/x97U0y3.png

If you’re talking about something totally different, just let me know.