Quickly, in this article, you are going to efficiently learn how to install cyanogenmod 12.1 Android Lollipop on ASUS Zenfone Go.
The entire process should work like a charm for you if you follow the procedure correctly. But DWYOR (Do it with your own Risk) I'm not responsible if you damaged your phone :)
Asus ZenFone 5 will not upgrade-able to Android Marshmallow based on ASUS Android 6.0 Update plan, but there are a lot of developer out there still try to support this device. A couple days ago dgadelha, senior member xda developer create an experimental build CyanogenMod 13 for ASUS ZenFone 5. This ROM is experimental build so there are a lot of bugs, but it's a huge step for ZenFone 5 development. The original thread is Here.