Locate the button on the back of the clock.
Long press the button for 2 seconds. Let go when the hours digit starts flashing.
Click the button to change the digit.
Long press the button for 2 seconds to move to the next digit. (The minutes digits are set separately.)
Click the button to change the digit.
Repeat for the last digit.
Long press the button again to return to the regular clock mode.
Unplug vs Turn off switch
If you unplug the clock, it will stop tracking time, but the time it was unplugged is stored and loaded next time you plug it in. So if you unplug it and replug 5 minutes later, it will be 5 minutes behind.
If you turn off the on/off switch, only the LEDs turn off, not the whole system. If you turn the switch off and leave the USB cable plugged in, the controller will still be powered, and it will still keep accurate time.