Which model tablet are you using? And are you looking to do a full factor reset or just a soft reboot?
If you would like to just reboot the tablet (off then on):
If it is working, pull down the notification window all the way, look for the power button on the top of the and you will get the option for power off or reboot.
If you are looking to reset a frozen device, hold the physical power button and the volume down button until the device resets itself.
