Use a Jquery selector to find the elements by the content of the data- attribute. Then use attr() to alter the value
$('input[placeholder = "Your Email Address"]').attr('placeholder', 'Write email');
$('input[value = "Subscribe"]').attr('value', 'Send');
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div class="gdlr-core-newsletter-item gdlr-core-item-pdlr gdlr-core-item-pdb  gdlr-core-style-rectangle" style="padding-bottom: 0px ;"  id="mynwsltr"  >
<div class="newsletter newsletter-subscription">
   <form class="gdlr-core-newsletter-form clearfix" method="post" action="/?na=s" onsubmit="return newsletter_check(this)">
      <div class="gdlr-core-newsletter-email"><input class="newsletter-email gdlr-core-skin-e-background gdlr-core-skin-e-content" placeholder="Your Email Address" type="email" name="ne" size="30" required /></div>
      <div class="gdlr-core-newsletter-submit"><input class="newsletter-submit" type="submit" value="Subscribe"  /></div>
   </form>
</div>
 
 
This can be re-written as plain Javascript using:
A querySelector instead off $()
setAttribute() instead off Jquery's attr()
document.querySelector('input[placeholder = "Your Email Address"]').setAttribute('placeholder', 'Write email');
document.querySelector('input[value = "Subscribe"]').setAttribute('value', 'Send');
<div class="gdlr-core-newsletter-item gdlr-core-item-pdlr gdlr-core-item-pdb  gdlr-core-style-rectangle" style="padding-bottom: 0px ;"  id="mynwsltr"  >
<div class="newsletter newsletter-subscription">
   <form class="gdlr-core-newsletter-form clearfix" method="post" action="/?na=s" onsubmit="return newsletter_check(this)">
      <div class="gdlr-core-newsletter-email"><input class="newsletter-email gdlr-core-skin-e-background gdlr-core-skin-e-content" placeholder="Your Email Address" type="email" name="ne" size="30" required /></div>
      <div class="gdlr-core-newsletter-submit"><input class="newsletter-submit" type="submit" value="Subscribe"  /></div>
   </form>
</div>