You can change the twitter text by adding the data-text attribute to the button.
If you haven't overriden your template please do. This post will help you.
nuevvo.com/blog/item/84-k2-inheritance-sub-templating
Locate the twitter button
<?php if($this->item->params->get('itemTwitterButton',1)): ?>
<!-- Twitter Button -->
<div class="itemTwitterButton">
<a href="https://twitter.com/share" class="twitter-share-button" data-count="horizontal"<?php if($this->item->params->get('twitterUsername')): ?> data-via="<?php echo $this->item->params->get('twitterUsername'); ?>"<?php endif; ?>>
<?php endif; ?>
and add the attribute
<?php if($this->item->params->get('itemTwitterButton',1)): ?>
<!-- Twitter Button -->
<div class="itemTwitterButton">
<a href="https://twitter.com/share" class="twitter-share-button" data-text="<?php echo $this->item->title; ?>" data-count="horizontal"<?php if($this->item->params->get('twitterUsername')): ?> data-via="<?php echo $this->item->params->get('twitterUsername'); ?>"<?php endif; ?>>
<?php endif; ?>
All the available attributes can be found here:
dev.twitter.com/web/tweet-button 
