You cannot. Only removing the device from the charger will power off the device.
There is no way to manually power off the devices, and there is never a need to. As long as you are charging your devices consistently after every use, they will automatically power off once pulled off the charger. We recommend leaving them off until powering them on at the field by simply pushing the button.
If a reset is needed to troubleshoot poor connection, tap and hold the front button for 10-15 seconds until the watch goes black and resets.