downgrading and installing marshmallow in Asus ze551ml and ze550ml this method will work in both devices

for asus ze 551ml and ze550ml firmware u willl need
you can download its programs which is used it this process from google;

asus zenfone 2 ze551 ml firmware

u can download firmware for asus site

now install intel andoid and adb drivers 

now download plateform tools

and install them in your pc

copy your firmware in plateform tools

before you do that u need to enable your pc signaure

hold shift key and presss resart buton

now tap on continue

after restart hit F7

now go to platform folder

hold shift and press right mouse button

click on open cmd here or open windows terminal here

and connect tour phone with pc via usb cable

and go to recovery mode from your devices

now write the following cmd in plateform tools before you make sure that your phone is connected with pc

adb devices 

and hit enter

if device has been shown its mean that you mobile has been connected with pc successfully if not try again that command if still you are having same problem than its mean that you did not install any thing correctly it may be your drivers or anything else

copy the entire name of your firmware without .zip extension and don't use paranthesis brackets it is just for demonstration;

adb sideload (firmware name).zip

now wait until it will insalled in your device

now you firmware has been install 100% successfully

if you face any error while installing your firmware its mean that you did't download your mobile's firmware correctly now try flashing another firmware in you device 

Thanks To All

created and posted by Malik's brothers

and enjoy