How to know where to override vendor methods

Look into binding classes into the service container. You'd bind your class instead of the original.

An example

public function register()
{
    $this->app->singleton(
        // the original class
        'Laravel\Spark\Contracts\Interactions\Settings\Profile\UpdateContactInformation',
        // my custom class
        'App\Http\SparkExtensions\UpdateContactInformation'
    );
}

That basically tells laravel "whenever an instance of Laravel\Spark.....\UpdateContractInformation (which is in /vendor/Laravel/Spark...) is called, give an instance of App\Http\SparkExtensions\UpdateContactInformation instead" https://laravel.com/docs/5.7/container

The custom App\Http....\UpdateContactInformation class extends the original Laravel\Spark.....\UpdateContractInformation class to inherit the methods, and then I just override what I need in the custom class.

@CRONIX - okay, but the problem that i'am facing now that i don't know where to write this binding exactly in the service provider ?

Yes, it's explained in the docs I linked to.

Almost all of your service container bindings will be registered within service providers, so most of these examples will demonstrate using the container in that context.

https://laravel.com/docs/5.7/container#binding

Then you can also read about the service providers it's referring to: https://laravel.com/docs/5.7/providers