In order to change the firmware, there are several options. You can reflash the phone using special software and reset all data which are not systemic. For this you will need to dial a special code using your cell phone.
If you want to reflash the phone, you will need to connect the phone to PC using special usb wire that is included with the phone. If not, buy it separately. Use the program for updating the firmware and the firmwarethat fit your model of phone. Don't forget to keep the firmwareon your phone before flashing. Instructions for flashing, a program for flashing, and also the very firmware you will easily be able to find on the Internet.
If you decide to reset the firmware, contact the manufacturer of your cell phone. Do not use combinations, which you can find on the Internet - in this case, all risk for misuse of the codes are on you. Using code obtained from the manufacturer, reset the firmware and enjoy a fully clean phone.