How to edit original wordpress HTML source code and add icon?

Hello I want to know how to edit wordpress html and add custom html
for example I want to add html element like icon beside a link. here is the code:

wordpress original source code:

<a rel="nofollow" class="comment-reply-link"> Reply </a>

I want to edit this wordpress html code to add this icon to be:

<a rel="nofollow" class="comment-reply-link">
<i class="material-icons"> Reply </i>
</a>

UPDATE:
another example to be clear:
I want to add custom id to the link
which is wordpress blog comment element source code not a theme source code.
please, how to do that?
and many thanks in advance