Firstly, create a Child Theme for your WordPress Theme.
Then look in '/wp-content/themes/[theme-name], where you should find a file entitled 'single.php'. Copy this into your child theme, taking care to ensure you replicate the same directory hierarchy. The 'single.php' is usually the name of your default Blog Post Template.
Open up the 'single.php' file, which you have saved within your child theme, with a programme such as Notepad++ (Notepad will also do but is not as easy on the eye).
You should see something like:
<?php if ( has_post_thumbnail() ) {
the_post_thumbnail();
}?>
<?php
if (have_posts()) :
while (have_posts()) : the_post(); ?>
<h1><?php the_title(); ?></h1> //This is the title of your Blog Post.
<div class="entry">
<?php the_content(); ?> //This is the content of your Blog Post.
</div>
<?php endwhile; ?>
<?php endif; ?>
There will be a slight variation in the coding but what you are looking to do is highlight:
<?php if ( has_post_thumbnail() ) {
the_post_thumbnail();
}?>
and then move this coding to beneath <h1><?php the_title(); ?></h1>
. You will need to play around as to where exactly you will need it but I hope this helps get you started.
The reason I have suggested that you do this within a Child Theme is that when the Theme Developer rolls out an update, it will delete any modifications you have made within the parent files.
Good Luck!