Look for the VirtualHost blocks in /etc/apache2/sites-enabled that you want to set as the default. Make the following change(s)

<VirtualHost  *:80>   or <VirtualHost *:443>

to

<VirtualHost _default_:80>   or  <VirtualHost _default_:443>

Make sure only one virtual host block is assigned the default (max one default per port). As you may already know, http URLs are usually assigned to port 80 and https URLs to port 443.