1. I assume you already have android usb drivers and also Adb installed on your PC , if so move to the next step :
2. download and install Asus flash tool (AFT)
http://www.mediafire.com/file/ucbdh7bj91patpd/ASUS.Flash.Tool.v1.0.0.34.zip
3. Download Asus RAW firmware 13.8.26.92
http://www.mediafire.com/file/2d4qxprvlkimovd/WW_ZC550KL(8916)-13.8.26.92_M3.1.42_Phone-user.raw.zip
4. boot the device into flashboot mode (csc mode)
5. open Asus flash tool and connect your device via usb cable to the PC , after connecting , the serial number of your device will appear in AFT .
6. In AFT , go to the "model" option and choose "ZC550KL" from the dropdown menu
7. also in AFT click a yellow button at the top (it opens the windows explorer) and browse for the Raw file you downloaded in step №3 (do not unpack)
8. also in AFT , on the "WIPE DATA" option , tick the "YES" option
9. now use you mouse to click or just highlight the serial number of your device that is being displayed in AFT and then click the "START" button.
10. the flashing process will start , it might take up to 30-40 or less , when the process is finished , you'll see a SUCCESSFULLY displayed , but in some cases AFT might keep on running while in actual sense , the flashing process is already completed, just check if your device has rebooted , if it has , then just exit AFT.