First, in config/app.php, make sure these 2 providers are in this order. It doesn't matter where in the providers array they appear, as long as SparkServiceProvider comes before RouteServiceProvider:
Laravel\Spark\Providers\SparkServiceProvider::class,
App\Providers\RouteServiceProvider::class
Then you can just override any spark-specific routes with your own routes in the /routes/api.php or routes/web.php files. Just put your custom routes at the top and use the same url that Spark is using (unless you want to have to edit all of the views and js files using those urls since they're hardcoded) but change the controller/method it's using to yours.
As always, you will have to clear cache in between changes if your routes/config is cached.