Remove sticky behavior from top navigation on twenty fourteen theme?

Before I answer this question, I would believe that you have already created a child theme. Reason: You should never make any changes to a theme/plugin that you are not the author of. This includes core files

With that sorted, the behavior of the navigation menu in purely controlled by js,

  77          /*
  78           * Fixed header for large screen.
  79           * If the header becomes more than 48px tall, unfix the header.
  80           *
  81           * The callback on the scroll event is only added if there is a header
  82           * image and we are not on mobile.
  83           */
  84          if ( _window.width() > 781 ) {
  85              var mastheadHeight = $( '#masthead' ).height(),
  86                  toolbarOffset, mastheadOffset;
  87  
  88              if ( mastheadHeight > 48 ) {
  89                  body.removeClass( 'masthead-fixed' );
  90              }
  91  
  92              if ( body.is( '.header-image' ) ) {
  93                  toolbarOffset  = body.is( '.admin-bar' ) ? $( '#wpadminbar' ).height() : 0;
  94                  mastheadOffset = $( '#masthead' ).offset().top - toolbarOffset;
  95  
  96                  _window.on( 'scroll.twentyfourteen', function() {
  97                      if ( _window.scrollTop() > mastheadOffset && mastheadHeight < 49 ) {
  98                          body.addClass( 'masthead-fixed' );
  99                      } else {
 100                          body.removeClass( 'masthead-fixed' );
 101                      }
 102                  } );
 103              }
 104          }

which you can find in functions.js in the js folder

To remove this functionality, (all in your child theme) you’ll need to dequeue (using wp_dequeue_script()) the original functions.js, copy functions.js to your child theme and remove the mentioned lines and enqueue (using wp_enqueue_script()) the modified js. This will be done inside a custom function that will be hooked to the wp_enqueue_scripts hook

In code and in practice

• Create a file called js in your child theme’s root folder.

• Copy functions.js and paste it into your js folder in your child theme

• Open functions.js and remove the line I’ve mentioned above

• Safe the functions.js

Now, open your child theme’s functions.php, and add the following code in there. This will dequeue the parent script and enqueue your new modified script

function enqueue_child_functions_js() {

    wp_dequeue_script( 'twentyfourteen-script' ); //dequeue the parent script
    wp_enqueue_script( my-child-script', get_stylesheet_directory_uri() . '/js/functions.js', array( 'jquery' ), '20140717', true ); //enqueue new modified script

}

add_action( 'wp_enqueue_scripts', 'enqueue_child_functions_js', 999 );