That looks like a http://para.llel.us/support/ theme. You might start there for docs—for instance, they probably have a custom ‘Theme Options’ page in the admin.
Short of that, and presuming you’re ok with setting it in code (which may negate any future settings in the admin), you’ll first need to find out where $blog_query is being defined. You can start in functions.php but it’s hard to say without knowing the theme.
Once you find it, either update its posts_per_page value to the value you want or add it manually if it’s not there.