• Intel USB Driver (Download)
• ASUS Flashtool v1.0.0.11 (Download)
• ASUS Zenfone C ROM (Download)

Steps:

• Get your Zenfone C to boot into fastboot mode (Power button + Volume down)
• Connect Zenfone C to your PC and install USB drivers if not done already. The phone should show up as an “ASUS Android Composite ADB Interface” in the Device Manager.
• Now, open ASUS Flash Tool :- ASUS Flash tool v1.0.0.11 It should detect your phone with serial number and blue orb in front of it.
• Select your phone model i.e. ZC451CG Choose if you want to wipe data or not.
• Click box icon and browse for the signed zip image file you downloaded earlier.
• Select your device on the ASUS flash Tool by clicking on the serial number
• Select the device on the ASUS Flash Tool Click on the green start button, be patient, stay calm and cross your fingers. This could take 10-20minutes or so.