it's very easy. open the desired theme folder and edit the file single.php with your favorite text editor.
search for the <h2> post title:
now replace h2 with h1. the result would be:
<h2><?php the_title(); ?></h2>
<h1><?php the_title(); ?></h1>
Note: don't forget to edit the <h1> header for Blog Title in header.php.
Because if you don't edit it you will have two <h1> tags and that is not recommended at all.
<h1><a href="<?php echo get_option('home'); ?>/"><?php bloginfo('name'); ?></a></h1>