Filter Widget Title Wrap

The filter to do this is dynamic_sidebar_params also see this tutorial on this filter at ACF’s site (even if you don’t use ACF).

function prefix_filter_widget_title_tag( $params ) {

    $params[0]['before_title'] = '<h2 class="widget-title widgettitle">' ;

    $params[0]['after_title']  = '</h2>' ;

    return $params;

}
add_filter( 'dynamic_sidebar_params' , 'prefix_filter_widget_title_tag' );