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 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