Default virtual host will be the first loaded one. If you have multiple virtual hosts in same file, first from file will be default. If you have them in multiple files, VHost in first file will be default.

File are sorted alphabetical, with files starting with digit having highest priority:

Example
000-default.conf
an.example.com.conf
example.com.conf

So let’s say you have enabled following hosts (you can see enabled hosts using ls -l /etc/apache2/sites-enabled):

Sample Output
an.example.com.conf
example.com.conf

You want example.com to be default but it isn’t.
Disable it using a2dissite:

sudo a2dissite example.com

Now rename it so it comes before an.example.com:

sudo mv /etc/apache2/sites-available/example.com.conf /etc/apache2/sites-available/000-example.com.conf

Enable it once again and restart Apache:

sudo a2ensite 000-example.com
sudo systemctl restart apache2

Try to visit site via IP, it should be working correctly this time.