Here is tutorial How to Update ASUS Phone to Marshmallow Manually
1) First, enable USB debugging from setting.
2) Download official Android 6.0.1 Marshmallow for Android Phone from above and Unzip it.
3) Download the latest version of Odin from above and unzip the zip file.
4) Run the Odin_v3.10.7.exe file on your PC.
5) Now Boot your Android Phone into Download Mode by press and hold ‘Power + Home + Vol Down‘ buttons.
6) Once you enter into Download mode, you see an ID: COM port turned blue and Added message in Odin message box means your device has recognized by Odin.
7) If ID: COM doesn’t show means your driver is not correctly installed or try it with different USB cable.
7) Now locate and add the firmware ‘.tar‘ file (you downloaded from above) to the AP/PDA tab in Odin.
8) Make sure that only these two options are tick “Auto Reboot” and “F.Reset Time“. (make sure Re-partition is NOT ticked)
9) Now click ‘Start‘ button and it will start flashing the file. Wait until you see ‘Pass‘ in Green Color on Odin screen.
That’s all this is the complete guide to Update Android Phone to Marshmallow Manually. Let us know if you have any questions by comment below.