HTML Bootstrap navigation menu to WordPress menu using wp_nav_menu

You have to edit the files in:

wordpress/wp-content/themes/twentyseventeen/template-parts/navigation/navigation-top.php

and then create a custom walker, try to take a look here.

Try in this way:

<?php
/**
 * Displays top navigation
 *
 * @package WordPress
 * @subpackage Twenty_Seventeen
 * @since 1.0
 * @version 1.2
 */

?>
<div class="container">
  <nav class="navbar navbar-expand-lg navbar-light" id="mainNav">
    <a class="navbar-brand js-scroll-trigger" href="#page-top">Logo</a>
    <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarResponsive" aria-controls="navbarResponsive"
      aria-expanded="false" aria-label="Toggle navigation">
      <span class="navbar-toggler-icon"></span>
    </button>
    <div class="collapse navbar-collapse" id="navbarResponsive">

    <?php

  wp_nav_menu( array(
    'container'      => false,
    'menu_id'        => 'istoby-nav',
    'menu_class'     => 'navbar-nav ml-auto',
    'items_wrap'     => '<ul id="%1$s" class="%2$s">%3$s</ul>',
    'theme_location' => 'top',
    'depth'          => 5,
    'fallback_cb'    => false,
    'walker'         => new Bootstrap_Menu_Walker()
  ) );

  class Bootstrap_Menu_Walker extends Walker_Nav_Menu {

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

      if ( isset( $args->item_spacing ) && 'discard' === $args->item_spacing ) {
        $t="";
        $n = '';
      } else {
        $t = "\t";
        $n = "\n";
      }
      $indent = ( $depth ) ? str_repeat( $t, $depth ) : '';

      $classes = empty( $item->classes ) ? array() : (array) $item->classes;
      $classes[] = 'menu-item-' . $item->ID;

      // Filters the arguments for a single nav menu item
      $args = apply_filters( 'nav_menu_item_args', $args, $item, $depth );

      // Filters the CSS class(es) applied to a menu item's list item element
      $class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item, $args, $depth ) );
      $class_names = $class_names ? ' class="' . esc_attr( $class_names ) . '"' : '';

      // Filters the ID applied to a menu item's list item element
      $id = apply_filters( 'nav_menu_item_id', 'menu-item-'. $item->ID, $item, $args, $depth );
      $id = $id ? ' id="' . esc_attr( $id ) . '"' : '';

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

      // it would be better to enter the class in Appearance -> Menus -> Screen Options -> CSS classes
      // $output .= $indent . '<li' . $id . $class_names .'>';
      $output .= $indent . '<li class="nav-item">';

      $atts = array();
      $atts['title']  = ! empty( $item->attr_title ) ? $item->attr_title : '';
      $atts['target'] = ! empty( $item->target )     ? $item->target     : '';
      $atts['rel']    = ! empty( $item->xfn )        ? $item->xfn        : '';
      $atts['href']   = ! empty( $item->url )        ? $item->url        : '';

      // Filters the HTML attributes applied to a menu item's anchor element
      $atts = apply_filters( 'nav_menu_link_attributes', $atts, $item, $args, $depth );

      $attributes="";
      foreach ( $atts as $attr => $value ) {
        if ( ! empty( $value ) ) {
          $value = ( 'href' === $attr ) ? esc_url( $value ) : esc_attr( $value );
          $attributes .= ' ' . $attr . '="' . $value . '"';
        }
      }
      $title = apply_filters( 'the_title', $item->title, $item->ID );

      // Filters a menu item's title
      $title = apply_filters( 'nav_menu_item_title', $title, $item, $args, $depth );

      $item_output = $args->before;
      $item_output .= '<a class="nav-link js-scroll-trigger"'. $attributes .'>';
      $item_output .= $args->link_before . $title . $args->link_after;
      $item_output .= '</a>';
      $item_output .= $args->after;

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

  ?>

    </div>
  </nav>
</div>

For more information look here on:

• Walker_Nav_Menu
• Walker_Nav_Menu::start_el()

You can also edit the file in:

wordpress/wp-content/themes/twentyseventeen/header.php

Have you ever thought of starting from a blank theme like understrap?