You can use an Event Dispatcher. Setup the Dispatcher in your Slider BP (I assume this is an actor in the world the player character interacts with? Or is it a HUD element?), give it a name like ED_LightSlider.
Go into your Light BP, here you are going to create a Custom Event, name it BindLightSlider, off this right click and search for your ED_LightSlider, choose Bind Event. Next, create another Custom Event, name this Update_LightIntensity, link this to the Bind Functions delegate pin.
Up at your Begin Play, place a BindLightSlider call, this will call the Bind Function and activate it so it will start ‘Listening’ for calls.
Now when the ED_LightSlider (Event dispatcher in your Slider BP) is called, it wall trigger the Update_LightIntensity event.
You should now have a way to trigger events across all your placed Lights without having to specify which instance in the world you want to adjust. Another method would be to use a Loop that get’s all the light actors and applies the Light intensity adjustments from within the Slider.