turn the device off
press both softkeys ( the 2 keys directly under the screen, used to open menu's etc. )
briefly press the power button, 1 to 1.5 sec.
keep holding down the softkeys.
after about 1 second, you should see the bootsplash screen for about 1 second,
then you a black screen with 'press 0 to restore factory defaults'
pressing '0' now will hard reset your device.
note that the hard reset takes quite a while, several minutes.