Use your FTP program (something like FileZilla) and navigate to /plugins/system/IsRequiredVmCustom

Edit the file: IsRequiredVmCustom.php
Search for “Select an option”

It will located on/ or around Line 39
$matches[1] = implode('<option value="">Select an option</option><option', explode('<option', $matches[1], 2));

You can change the wording “Select an option” to anything you would like and it will be displayed on the frontend.