First of All, Welcome to my BLOG On this Thread, you will find the Original link to Download Lenovo Z2 Stock ROM Firmware from Google Drive. Lenovo Z2 Firmware Package comes in a zip package, which contains Flash File and Flash Tool and Driver with How-to Flash Manual.
Device Name: Lenovo Z2
Android Version: 4.4 KitKat
Install via: SP Flash Tool
File Size: 1.0 GB
File Hosted: Google Drive
How to Flash Lenovo Z2 Stock Firmware
Step 1: Download & extract the Firmware on your computer.
Step 2: After extracting Firmware zip file you will find the Driver and Installation Instructions.
Step 3: Install Latest available USB Drivers (Just skip this Step if your Computer already have).
Step 4: Follow the instruction to flash the firmware your Lenovo smarthphone.
Download Lenovo Z2 Firmware Update