Add the following script to DIVI > Theme Options > Integrations > Head Code:

 

<script>
jQuery(function($) {
window.et_pb_smooth_scroll = function( $target, $top_section, speed, easing ) {
var $window_width = $( window ).width();
$menu_offset = -1;
var headerHeight = 143;
if ( $ ('#wpadminbar').length && $window_width <= 980 ) {
$menu_offset += $( '#wpadminbar' ).outerHeight() + headerHeight;
} else {
$menu_offset += headerHeight;
}
//fix sidenav scroll to top
if ( $top_section ) {
$scroll_position = 0;
} else {
$scroll_position = $target.offset().top - $menu_offset;
}
// set swing (animate's scrollTop default) as default value
if( typeof easing === 'undefined' ){
easing = 'swing';
}
$( 'html, body' ).animate( { scrollTop : $scroll_position }, speed, easing );
}
});
</script>