How to serialize changes to Wdfb_OptionsRegistry

I am trying to set options myself with code.

I figured out how to get model and alter it like this:

$facebookUlt = Wdfb_OptionsRegistry::get_instance();
$facebookUlt->set_option('wdfb_api', 'app_key', $new_value);

But I cannot figure out how to serialize the changes back to options table. There is a lot of code in this plugin. Can you tell me where the code is that serializes the settings please.