overriding responsive navbar

Modality uses Bootstrap for responsive design. I want the hamburger menu drop down that appears in mobile view to persist regardless of display size (don’t want the horizontal menu) but I don’t want to override all responsive actions i.e. changing break point. Been to bootstrap but have not been able to figure it out yet. This is only my second wordpress site so I am a newbie. Here is the site artfulcoach.com. Not Pretty right now cause I am still learning. Any help would be appreciated