Editing functions/gallery.php

Hi
I am new to WordPress and unable to get theme provider to deal with me as a newbie.
I am trying to get the gallery/portfolio to accept a Youtube embed code as opposed to a URL.

The response to the ticket is as follows.
This can be accomplished by a code tweak – if you would like to try it, modify line 258 of the functions/gallery.php file:
1 $html.=pexeto_get_video_html( $video_url, $width );
to be like this:
1 $html.='<div class=”video-wrap”>’. $video_url .'</div>’;

My reply was “I’m out of my depth here. I would appreciate step by step details. The reply
“You can modify the file locally in your computer, using any text editor. The file can be found inside the .zip file of the theme that you installed. You can unzip it, and you will find the file there. Once you modified it, you can upload it to your server using FTP, replacing the original version of it.”
Perhaps I’m too stupid to be trying this but it still does not make sense.

Any chance someone can hold my hand, please.

Thanks