The solution ( if this is what you want ). This will make your subcategory 
  titles in the subcategory block, links, if you're using the default template 
  in your site. ( I'm still using version 701 btw - not the current 806 upgrade,
  but it may be the same ) 


  1. GO TO YOUR SITE VIA FTP, MAKE A BACKUP OF 
     yoursite.com/components/com_k2/templates/default/category.php in case something goes wrong.

  2. AROUND LINE 71 ( I USE THE FREE NOTEPAD++ EDITOR ), IN CATEGORY.PHP AND UNDER 
   '<---Subcategory title-->' ,
   
   
   YOU SHOULD SEE
   
   <h3 class="itemListCategoryTitle"><?php echo $subCategory->name;?> <?php if ($this->params->get('subCatTitleItemCounter')) echo '('.$subCategory->numOfItems.')';?></a></h3>


   AND REPLACE WITH

   <h3 class="itemListCategoryTitle"><a class="itemListCategoryTitle" href="<?php echo $subCategory->link;?>"><?php echo $subCategory->name;?> <?php if ($this->params->get('subCatTitleItemCounter')) echo '('.$subCategory->numOfItems.')';?></a></h3>

  IMPORTANT :( the above code should be all in one line, not like here above in this notepad txt file format as 3 lines )

   
   
   IF YOU WANT TO THEN CHANGE THE FONT SIZE OF THE TITLES, YOU HAVE TO GO TO

   yoursite.com/components/com_k2/css/style.css


   AND LOOK FOR AROUND LINE 331

   div.itemListCategoryBlock h2.itemListCategoryTitle a {font-family:Georgia, "Times New Roman", Times, serif;font-size:20px;}


   AND CHANGE THE 'font-size:20px' TO WHATEVER FONT-SIZE YOU WANT.

   
   IT WORKED FOR ME, AND I'M JUST A CUT AND PASTER, BUT MAKE BACKUPS OF ANY FILES YOU CUSTOMISE, FIRST, ALWAYS.
   
   HOPE IT WORKS FOR YOU, P