To do this you will need to include WP Bootstrap Navwalker in your functions.php which can be downloaded here, simple add:-

in your functions.php.

Now you will want to edit your header.php file so your menu looks something like:-

    <div id="nav">
    <nav class="navbar navbar-default navbar-fixed-top" role="navigation">
      <div class="container">
        <!-- Brand and toggle get grouped for better mobile display -->
        <div class="navbar-header">
          <button type="button" class="navbar-toggle" data-toggle="collapse" data-target=".navbar-collapse">
            <span class="sr-only">Toggle navigation</span>
            <span class="icon-bar"></span>
            <span class="icon-bar"></span>
            <span class="icon-bar"></span>
          </button>
          <a class="navbar-brand" href="<?php bloginfo('url'); ?>"><img decoding="async" class="logo" src="<?php bloginfo('template_directory'); ?>/images/logo.png" alt="" /></a>
        </div>


        <?php
        wp_nav_menu( array(
                'menu'              => 'primary',
                'depth'             => 2,
                'container'         => 'div',
                'container_class'   => 'navbar-collapse collapse',
                'menu_class'        => 'nav navbar-nav',
                'fallback_cb'       => 'wp_bootstrap_navwalker::fallback',
                'walker'            => new wp_bootstrap_navwalker())
            );
        ?>
      </div><!-- /.container-fluid -->
    </nav>
</div><!-- #nav -->

As long as you have included your bootstrap (CSS/JS) files in your or through your functions.php file, you now have a working bootstrap nav menu in your WordPress Theme.

You can include the Bootstrap core CSS & JS files from CDNJS:-

https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/4.0.0-alpha/js/bootstrap.min.js

https://cdnjs.cloudflare.com/ajax/libs/jquery-ui-bootstrap/0.5pre/assets/css/bootstrap.min.css