Can I remove the date from one post only?

Open index.php and replace

<h3><?php the_time('F jS, Y') ?></h3>

with

<?php if (!is_category('about')) { ?><h3><?php the_time('F jS, Y'); ?></h3><?php } ?>

You can check Oozman’s Date Remover plugin. https://oozman.com/oozing/ideas/archives/oozman-date-remover-wordpress-plugin

It’s working great. And you can optionally select whether to hide dates on homepage only or on posts only.

How do I open index.php?

You can open any template or any other file in a simple text editor. Notepad ++ is good for Windows.