Light Mask that reveals two different backgrounds

Hello, I am not super technical so I will do my best to explain. It's a 2D game where the player navigates the dark with a light source. The background in the dark looks one way and in the light looks completely different.As they move about the area that the light mask reveals the "lit" background. I can create the mask in photoshop easily but translating it to Unity is a bit difficult. I've seen the basic masking tutorials but it falls short of what I need. Any point in the right direction would be awesome.