First, you download the firmware for your phone from the forum. To find firmware, just check threads in the home section or mention your model number so I can send the link you want. After downloading, go to the file downloaded and name it update.zip if it has any other name. Then paste this file in the internal storage of your phone. Click on it, check clear data and click update now. This will flash the new firmware. Don't forget to backup your data with data backup & restore under storage & backup in settings before flashing the new firmware.