i tried to use https://fontawesome.com/ but couldnt success.. any help how to edit and change category icon in Segma theme..

Try these:
https://fontawesome.com/v5/search?o=r&m=free

Whats then...
i tried this: https://fontawesome.com/v5/icons/graduation-cap?f=classic&s=solid

this value is not working : f19d
this value is not working : graduation-cap
this value is not working : fa-graduation-cap

What should i enter in icon field?

fas fa-graduation-cap

If you mean the symbols on the home page, that would be the solution.

Download the plugin https://github.com/panaionutwteh/osclass/tree/master/category_icon first and then change the code to

main.php

<?php while(osc_has_categories()) { ?>
      <a href="<?php echo osc_search_category_url(); ?>">
          <div class="icon" <?php if(osc_category_color() <> '') { ?>style="color:<?php echo osc_category_color(); ?>;"<?php } ?>>
      <i class="<?php if(get_category_icon(osc_category_id())) { ?>"></i>
              <img src="<?php echo get_category_icon(osc_category_id()); ?>"/><br>
             <font size="2" color ="black"><b><?php }
                                       echo osc_category_name(); ?></b></font>
      </div> <style>
      img {
      width: 70px;
      height: 70px;
      }
    </style></a>


You can see what it can look like on my website. https://wytery.com