Hi there!

Please use the following code in your custom CSS:

@media (max-width: 767px){
#section-id-1609319516677 .sppb-col-xs-6 {
    flex: 0 0 100%;
    max-width: 100%;
}
}

Note: Path towards custom CSS: Extensions>Templates>Styles> Click on your current default template name then choose Template Options. Then find custom CSS within the custom code.

Best Regards