How To Customize Header Button (WordPress Website Astra Theme)

<div class="tnp tnp-subscription"> <form method="post" action="https://www.dienodigital.com/?na=s"> <input type="hidden" name="nlang" value=""><div class="tnp-field tnp-field-firstname"><label for="tnp-1">First name or full name</label> <input class="tnp-name" type="text" name="nn" id="tnp-1" value=""></div> <div class="tnp-field tnp-field-email"><label for="tnp-2">Email</label> <input class="tnp-email" type="email" name="ne" id="tnp-2" value="" required></div> <div class="tnp-field tnp-field-button"><input class="tnp-submit" type="submit" value="Subscribe" > </div> </form> </div>