Is it possible to extend an existing contributed module?
If you're using core's configuration sync to manage your site's config, you should be able to update this through the UI, then export your site's configuration to the sync directory, e.g. drush cex sync. This will create a new file in the config sync directory for the updated config which can be safely committed along with your other changes. While deploying, you'll need to import the synced configuration, e.g. drush cim sync to import the UI change on your second environment.
Configuration files in a module's config/install directory are only read from that location when the module is installed. That configuration file shouldn't change at all here and there's no reason for it to.