Factory data reset will erase all your data and apps. You will get a “new” phone in factory state. So, backup, backup, backup your data and files. You can use Kies 3 to backup most data into PC. Once you backup all your data, you can do a factory data reset in Settings— Backup and reset — Factory data reset . Please note, factory data reset will NOT bring you back to KitKat . You will still on Lollipop. If you really want to roll back to KitKat after Galaxy Note 4 Android Lollipop update, you need visit local Samsung service center. They may help you flash back the old ROM. Do NOT do it yourself unless you want to brick your phone