How to change displayed category

Hi,

This is a really great theme, I would just like to change one thing – displaying categories on billboard and in Category archives – by default it displays random one (if a post is in more than one category) but I wish it would display the primary category.

It’s especially important for Category Archives, as on my site:
https://poszli-pojechali.pl/ – as you click at one of categories in the menu, the template fetches title category from the first post, and if you click at the menu, for example, on Europa-W?ochy, it displays title: Category Archives: Na Wyrywki, which is a secondary category for the latest post.

I understand, that it’s this part of the code, in category.php:
<h1 class=”page-title”><?php _e( ‘Category Archives:’, ‘justwrite’ ) ?> <?php ac_output_first_category( ”, true ) . ac_icon( ‘angle-down’ ); ?> </h1>

I guess I should replace this: ac_output_first_category, but have no idea with what.

Best regards
Michal