With the tablet switched off, press the Power button and the Down Volume button together. Thhe device should open to a menu of seven items and the last entry is Factory reset.
That is to make sure no-one can accidentally reset the device by accident and that is because it will delete all your personal files - music, photos, etc.
To get to that menu entry, you need to press the Volume Down button seven times until the Factory reset is highlighted. Press the Power button at that stage and the device will close down and when it comes on again, the process will start.