Just checked, it's quite easy. Open "Settings" (the gear icon on top of the home screen) then go to "Device preferences" -> "Screen saver"

This may not be helpful as it applies to my Shield but Settings > Device Preferences > Screen Saver. Look for something similar?

If You want to setup Google Photos screensavers I've had zero luch on our Mi Box S. Neither by using the built-in settings or the Google Home app.