They are installed in pairs and neither has an on or off position marked on the switch. You should be able to turn the light fixture on or off from either end of the staircase.

Go to the main electrical panel and flip off the circuit breaker that’s feeding power to the light switch. (Or, unscrew the fuse if your home has a fuse box.) ...

What is a smart switch? These light switches connect to your Wi-Fi network and give you many extra features for controlling your house or office lights.