How to override default meta tags?
In Drupal 8, there is a [module-name].module file inside your module. You can use "Hook attachments alter".

Code to alter your meta tags -

// For adding description in meta tag .

function hook_page_attachments_alter(array &amp;$attachments) {
   $attachments['#attached']['html_head'][] = array(
        array(
          '#tag' =&gt; 'meta',
          '#attributes' =&gt; array(
            'name' =&gt; 'description',
            'content' =&gt; 'Any thing in meta description',
          ),
        ),
        'description',
      );

// For adding title in meta tag.

$attachments['#attached']['html_head'][] = array(
        array(
          '#tag' =&gt; 'meta',
          '#attributes' =&gt; array(
            'name' =&gt; 'title',
            'content' =&gt; 'Any thing in meta title you want',
          ),
        ),
        'title',
      );

// For adding noindex in meta tag-robots .

$attachments['#attached']['html_head'][] = array(
        array(
          '#tag' =&gt; 'meta',
          '#attributes' =&gt; array(
            'rel' =&gt; 'robots',
            'href' =&gt; 'noindex',
          ),
        ),
        'noindex',
      );

You can add more meta tags for different pages and can validate them too.