Add Class in anchor tag and <ul>in WordPress Menu

1. Add class in Anchor tag

Add following code in function.php

function add_menuclass($ulclass){
return preg_replace('/<a /', '<a class="nav-link"', $ulclass);
}
add_filter('wp_nav_menu', 'add_menuclass');

Output will like

2. Add class in UL tag in WordPress Menu

add following code in header.php for call the menu on wordpress

For the add class in UL
<?php
wp_nav_menu(array(
   'theme_location' => 'primary',
   'menu_class'     => 'navbar-nav mr-auto',
));
?>