From the home screen on your phone, press and hold the power button down on the top of the device. After a few seconds a dialog will pop up that gives you the options to turn off the power on your phone, restart etc. To restart the M8 into safe mode touch and hold the "Power Off" button for a few seconds and this will give you the prompt to power into safe mode.

1. Press and hold the power button
2. Hold down the "Power Off" option on the screen.
3. Click OK on the dialog that pops up to reboot in safe mode.