change admin directory ?

If I understand you correctly, you can just define a route (e.g. in your theme's service provider) from whatever you want to the admin, such as:

protected $routes = [
        'custom-admin-uri' => 'Anomaly\DashboardModule\Http\Controller\Admin\DashboardsController@index'
    ];

Then yoursite.com/custom-admin-uri will take you to the admin's dashboard. There might be a more recommended way to do it though...

More on service providers and routes here: https://pyrocms.com/documentation/streams-platform/v1.1#core-concepts/service-providers

You can not globally change the admin directory for many many reasons. However you can create "admin" areas just like @samharrison7 mentions anywhere by simply extending the AdminController in the Streams Platform with your own controller.