You will need 2 buttons and a barrier
step 1. change the barrier color to pitch black and turn off collision and borders.
step 2. place the buttons next to each other.
step 3. using the buttons, make 2 channels, light on and light off.
step 4. make it so when the button connected to light on is pressed, it deactivates both the light on button, and the light off barrier.
step 5. do the same with light off, but instead of deactivating the barrier, it activates the barrier instead.
step 6. make it so when light on is pressed, it activates light off, and make it when light off is pressed, it makes it where light on is activated.
Now, that may seem confusing, so here it is in pictures