Unfortunately, it appears that there is no easy way to change the width that triggers the menu switching between mobile and desktop. I asked the developer who worked on it, and he says that it would require editing multiple files, as there's not only a lot of CSS, but also a few different javascript functions involved in the process.
Since it would require custom coding (and a lot of it), the best I can do is suggest that you post to the
Hire-a-Coder area to see if someone else is able to help you do it.