How to change submenu to drop down instead of right?

I currently have:
-Nav bar
-topics
-subtopic year
-subtopics
-sub sub topic

I want to make the “subtopic year” submenu go DOWN like an accordion rather than to the right. Currently, everything except the “topics” goes to the right which causes it to overflow off the screen.

I’m using SNO and I’m knowledgeable in CSS, but I can’t find how to change this functionality.