There are several reasons that you need to factory reset series, you will know the reason when you need to factory reset it. :)
These are the steps
1. Power down the device.
2. Keep the settings key and back key on the device pressed(the outer 2 keys at the bottom of the device) and power it up.
3. You will see that it powers on and a red LED next to the front camera will start blinking.
4. Once it starts blinking press the volume buttons + and - at the side of your device for 6 times alternately. i.e., + - + - + -
5. Once this sequence is pressed, you will see a solid red light for a while and this confirms that the factory reset is going on.
6. After the reset is done the device will boot up.