The Marlin has no power on off button. It autosleeps after 1 minute of inactivity, which is equivalent to turning it off.
Note that when it is connected to your smartphone, it is considered in use so you need to close the App before it is considered inactive. The blue light will blink when it is not connected to your smartphone.
After 1 minute, the blue light should stop blinking when it goes into sleep mode. The Marlin will announce it is going into sleep mode by saying "Sleep". To wake it up again, press the Start/Stop button and the Marlin say "Wake Up".