Hi
Go to /templates/ja_magz_ii/less/variable.less
Find this code :
// Point at which the navbar stops collapsing
@grid-float-breakpoint: @screen-sm-max;
Replace it with
// Point at which the navbar stops collapsing
@grid-float-breakpoint: 1024px;
Apply the value 1024px with the screen size where u want to apply the breakpoint .
Compile Less to css from template options to apply the changes . If you will edit this file via Joomla admin panel you can compile only this file from the compile Option . Here > http://prntscr.com/afsrxe
Please take a backup of site before making changes .