Asus Zenfone 2 was launched in March this year. It has a 5.5 inch display with a resolution of 720 x 1280 pixels. It is powered by a Intel Atom CPU which is clocked at 1.8GHz and has 4 cores. The rear camera is 13MP while the front camera is 5MP. The internal memory of Zenfone 2 is 16GB and it has a microSD card slot. It runs on Android Lollipop with Asus planning to provide official Marshmallow update.
It will take some time for Asus to deliver Marshmallow update for Zenfone 2. Till then you can use CM 13 ROM running on Android 6.0. CyanogenMod team has made CM 13 ROMs for Asus Zenfone 2 ZE550ML and Asus Zenfone 2 ZE551ML variants. A detailed guide on installing CM 13 on Zenfone 2 is given below.