Hello, @mileva_lukic

I want to change the title CATEGORY of the drop down menu at the top right side of the page. How?

To change title CATEGOTRY of the dropdown menu at the top right side of the page you need to do customization by creating child theme.
To create child theme reference here https://codex.wordpress.org/Child_Themes and don’t forget to activate child theme.
Now in functions.php of your child theme you need to add the following code:

function photo_perfect_add_category_navigation(){

    $show_category_dropdown = photo_perfect_get_option( 'show_category_dropdown' );
    if ( true !== $show_category_dropdown ) {
      return;
    }
    ?>
    <div id="category-menu" class="clear-fix header-navigation">
      <div class="container">
        <button class="nav-list-btn"><i class="fa fa-list"></i><span><?php _e( 'Your text', 'photo-perfect' ); ?></span></button>
        <div class="category-list-wrapper">
          <ul>
          <?php wp_list_categories( 'title_li=&depth=1' ); ?>
          </ul>
        </div><!-- .category-list-wrapper -->
      </div><!-- .container -->
    </div><!-- #category-menu -->
    <?php
  }

Hope this will help to resolve your issue.

Regards!!!