Update PHP Variable after Ajax call

I am making an Ajax call to a PHP file that checks a condition and runs a query a MySQL query if the conditions are met.

The query updates a table in my DB with a new value. This all works great. I would like to know how to show the new value in the current page without having to manually reload. Code is Below.

The variable I am updating is $trialExpiry

HTML/PHP

<h4 class="sbText mt-10">Trial End Date: <?php echo date("d/m/Y",
                        strtotime($trialExpiry)); ?></h4>

<form id='promocode'>
     <input type='text' class='sbInput' placeholder='Promo Code' name='promocode'>
     <input type='hidden' name='userid' value='<?php echo $userID; ?>'>
     <button class='btn sbSubmit'>Submit</button>
</form>

JQUERY

   <script>
         $(function () {
             $('#promocode').on('submit', function (e) {
                  e.preventDefault();
                  $.ajax({
                     type: 'post',
                     url: '../model/process-promo-code.php',
                     data: $('#promocode').serialize(),
                     success: function () {
                          $("button.btn").css({
                             'transition-duration': '1000ms',
                              'opacity': '0.5'
                           });
                     }
                 });
            });
      });
   </script>

Thanks so much.