Magento 2 : How to override a source model?

You can do it via plugin. First of all you must create di.xml

<config>
    <type name="Magento\Fedex\Model\Source\Generic">
      <plugin name="your_plugin_name" type="YourVendor\YourModule\Plugin\GenericPlugin" sortOrder="10" />
    </type>
</config>

Then you must create GenericPlugin.php and use after method.

namespace YourVendor\YourModule\Plugin;

class GenericPlugin
{
    public function afterToOptionArray(\Magento\Fedex\Model\Source\Generic $subject, $result)
    {
        // here you change the result
        return $anotherResult;
    }
}

More information here.