There is no button to turn the screen off instantly. However, all you have to do is wait! After ~5 minutes of inactivity Hydrow's screen will dim and then turn off automatically. Hydrow will then go into sleep mode until you wake it up by touching the screen or pulling the handle.
We do not recommend turning off the main power switch in order to accomplish this. For more details on why, see: Should I Power Hydrow Off Between Workouts?