Safe Mode puts your phone in a diagnostic state (returned to default settings) so you can determine if a third-party app is causing your device to freeze, reset or run slow.
An alternate method is available if the device is unresponsive or won't power on.
Press and hold the Power button (uppper-right edge) until the Power off option appears then release.
Touch and hold Power off until the 'Reboot to safe mode' prompt appears then release.
From the 'Reboot in safe mode' prompt, tap Turn on Safe Mode to confirm.
The process may take up to 30 seconds to complete.
Upon reboot, 'Safe mode' appears in the lower-left of the unlock/Home screen.