Default web site will launch when Apache cannot match the domain name user requested with one of the hosted domains.

To change the default web site in Plesk:

• Tools and Settings > IP Addresses
  
• Click on your IP Address
  
• Select a site from the Default site list that you want to be your default
  
• If you select None, Apache will load site located in /var/www/vhosts/default 

Plesk 17
CentOS 7