This theme fully supports default WordPress Customizer (with help from Kirki plugin). You can access it via Appearance / Customize, where you can edit almost every aspect of the theme like menus, sidebars and widgets, header logo and much, much more.

Customizer screenshot
Customizer screenshot

When you are trying to edit widgets via Customizer, only sidebars visible on your current page are available. Go to Appearance / Widgets to manage all sidebars in one place