class Main_Walker_Nav_Menu extends Walker_Nav_Menu {


function start_lvl( &$output, $depth ){ //ul
$indent = str_repeat("t",$depth);
$submenu = ($depth > 0) ? ' dropdown-menu' : '';
$output .= "n$indent<ul class="dropdown-menu$submenu">n";  }




function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 ){ //li a span


$indent = ( $depth ) ? str_repeat("t",$depth) : '';


$li_attributes = '';
$class_names = $value = '';


$classes = empty( $item->classes ) ? array() : (array) $item->classes;


$classes[] = ($args->walker->has_children) ? 'dropdown' : '';
$classes[] = ($item->current || $item->current_item_anchestor) ? 'active' : '';
$classes[] = 'menu-item-' . $item->ID;
if( $depth && $args->walker->has_children ){
$classes[] = 'dropdown-submenu';
}


$class_names =  join(' ', apply_filters('nav_menu_css_class', array_filter( $classes ), $item, $args ) );
$class_names = ' class="' . esc_attr($class_names) . '"';


$id = apply_filters('nav_menu_item_id', 'menu-item-'.$item->ID, $item, $args);
$id = strlen( $id ) ? ' id="' . esc_attr( $id ) . '"' : '';


$output .= $indent . '<li' . $id . $value . $class_names . $li_attributes . '>';


$attributes = ! empty( $item->attr_title ) ? ' title="' . esc_attr($item->attr_title) . '"' : '';
$attributes .= ! empty( $item->target ) ? ' target="' . esc_attr($item->target) . '"' : '';
$attributes .= ! empty( $item->xfn ) ? ' rel="' . esc_attr($item->xfn) . '"' : '';
$attributes .= ! empty( $item->url ) ? ' href="' . esc_attr($item->url) . '"' : '';


$attributes .= ( $args->walker->has_children ) ? ' class="dropdown-item dropdown-toggle"' : '';
$attributes .= ( !$args->walker->has_children ) ? ' class="nav-link"' : '';


$item_output = $args->before;
$item_output .= '<a' . $attributes . '>';
$item_output .= $args->link_before . apply_filters( 'the_title', $item->title, $item->ID ) . $args->link_after;
$item_output .= ( $depth == 0 && $args->walker->has_children ) ? '</a>' : '</a>';
$item_output .= $args->after;


$output .= apply_filters ( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );


}
}

<?php
wp_nav_menu(
array(
 'theme_location' => 'geo-header',
 'container' => 'ul',
 'menu_id' => 'mainNav',
 'menu_class' => 'nav nav-pills',
 'walker' => new Main_Walker_Nav_Menu(),
 )
);
?>