Changing home_content_boxes_layout

I see in basic-functions.php where the layout is controlled by adding the bootstrap classes for columns…

case $BoxCount == 4:
$BoxClass = ‘col-md-3’;
break;

I would like to change this to:

case $BoxCount == 4:
$BoxClass = ‘col-lg-3 col-md-6’;
break;

So that I get two rows of two on medium device widths. I have tested this by updating this file, but how do I change this in a child theme so that this change never gets overwritten by an update?

Thanks,
Scott