Bootstrap navbar-fixed-left side menu.

What would be the quick way to change the bootstrap 8.x-3.15 default horizontal menu to a vertical left side menu. The example I found, uses navbar-fixed-left. What would be the recommended way to implement this in a bootstrap sub-theme with minimal changes from the base theme?

Drupal version: 


Source: https://www.drupal.org/taxonomy/term/3/feed