Change tablet breakpoint responsiveness in Highend theme

Hello,
Our Highend theme menu does not collapse properly – to hamburger – when the browser is narrowed to tablet width. It finally does as it gets to mobile phone width, but the menu overlays the title banner in a very unattractive way until the browser finally gets so narrow that it pops into “hamburger” menu on mobile phone width.

I noticed that the Highend demo menu collapses properly, but my site doesn’t. Is there some CSS code you can offer me that will fix this breakpoint/responsive menu issue for tablet width? Thank you in advance for any help!