Tablet 570px Breakpoint Location Edit or Custom Code

Hi. Really impressed so far with Upfront Builder. One very basic question, obviously at present Upfront builder doesn't allow the editing of breakpoints for tablet / responsive mode.

What would be the best way to change the tablet breakpoint from 570px to say 640px? Editing a core file / setting or custom code in your opinion for best practice? And if so, where?

Thanks.