edit / components/com_k2/templates/default/item.php

look for the line of code n 354

and change this:

  <a title="<?php echo htmlentities($attachment->titleAttribute, ENT_QUOTES, 'UTF-8'); ?>" href="<?php echo JRoute::_('index.php?option=com_k2&view=item&task=download&id='.$attachment->id); ?>">
			    	<?php echo $attachment->title ; ?>
			    </a>
with this:

  <a title="<?php echo htmlentities($attachment->titleAttribute, ENT_QUOTES, 'UTF-8'); ?>"<a href="index2.php?option=com_k2&amp;view=item&amp;task=download&amp;id=<?php echo $attachment->id ?>">
			    	<?php echo $attachment->title ; ?>
			    </a>

Ciao.