Order posts in category by price

You can alter the loop with a query_post:
http://codex.wordpress.org/Function_Reference/query_posts
http://codex.wordpress.org/Function_Reference/WP_Query

Put something like this above the loop you want to sort by price:
query_posts( 'meta_key=price&orderby=meta_value_num' );

How can I do this? Editing some file…?

Yes, if you want to alter the order on category pages you need to put this code above the loop in your theme’s category.php (if your theme has one) or archive.php. And change the meta_key=price to your meta_key name. And don’t forget to put it inside php tags:

<?php query_posts( 'meta_key=price&orderby=meta_value_num' ); ?>

What theme are you using?

Because this is a premium theme and I cannot look in the theme’s template files it’s maybe better to ask support on their forums: https://www.elegantthemes.com/forum/