change when mobile navigation menu loads

hi

I am using opencart 2.0.2.0 with the default template.

I need to change the default template mobile navigation menu so it loads at media query 860px rather than the default at 767px. Its not in the default style sheet (CSS). I am guessing it something to do with a javascript file, perhaps the bootstrap file but I cant find anything referring to this in the js files.

I cant override this with CSS, if I look at the CSS in firebug I can see its not a css media query that is making this load and any media queries I make will not override it loading.

Can anyone advise please?