Change Classes in Nav-Tag of Custom Menu
You'll need to enable Twig template debugging output to do this. Once you do that you should see a number of template suggestions upward in the DOM for the &lt;nav&gt; tag. For example:

   * block--mainnavigation.html.twig
   * block--system-menu-block--main.html.twig
   * block--system-menu-block.html.twig
   * block--system.html.twig
   x block.html.twig


(Note this assumes you are using Drupal's block system to render the menu.) Above are a number of suggestions. If I look in a likely candidate from above, for example, block--system-menu-block.html.twig, I see markup like this:

&lt;nav role="navigation" aria-labelledby="{{ heading_id }}"{{ attributes|without('role', 'aria-labelledby') }}&gt;


You can grab any of these templates from core and then override by using a copy of the template in your theme and then alter the markup per your requirements. 

If you are using Twig Tweak to render the menu, then in that case, you can just add your own custom &lt;nav&gt; tag like so, usually in a page template such as page.html.twig:

  &lt;nav role="navigation" aria-labelledby="off-canvas-menu" class="main-menu" id="off-canvas-menu"&gt;
    {{ drupal_menu('main', 1, 4, TRUE) }}
  &lt;/nav&gt;