1) press "power" button for about 8 seconds. The button is located right next to Ethernet port.
2) execute command
Shutdown scheduled for Sat 2000-01-01 00:04:41 UTC, use 'shutdown -c' to cancel.
Broadcast message from root@beaglebone (Sat 2000-01-01 00:03:41 UTC):
The system is going down for power-off at Sat 2000-01-01 00:04:41 UTC!
Connection to 192.168.7.2 closed by remote host.
Connection to 192.168.7.2 closed.
WAIT until all LEDs are OFF and unplug the USB cord.