How to replace the "underline" set in css using a class within a p tag [closed]

I want to use a class element in a <span> tag that is underneath the <p> tag to replace the underline set in the parent-level <p> tag. Below is my code:

#instru_topp p span:last-child {
  text-decoration: underline;
}

<div id="instru_topp">
  <h3>The topping</h3>
  <p><span class="especial">Mix</span> all the -- <span>Ingredients</span> and pour it on the top cake. After bakin it, you will end up with this:</p>
</div>