You can easily download Android Oreo 8.1 for Xioami Mi Mix 2 and update it using OmniROM 8.1 Beta Custom ROM. Xiaomi Mi Mix 2 is no doubt a good budget friendly flagship device but sometimes companies are not able to provide support updates on time. At least not as fast Google provides latest updates for Pixel and Nexus devices. Mi Mix 2 is bezel-less device from Xiaomi that comes with 18:9 1080p display panel. Under the hood it carries Snapdragon 835, 8/6 GB RAM, 256 / 128 / 64 GB Storage and 12MP Back Camera with beautiful ceramic design.
Instead of competing with Samsung Galaxy S8 or Google Pixel 2 like flagship, it feels like Mi Mix 2 is more of OnePlus 5 replica, with almost same specs and price. There is no wireless charging, water proofing or even headphone audio jack. Not only this but also Xiaomi is unable to keep up with rival in terms of releasing Android updates. By default, Mi Mix 2 is running Android 7.1.1 Nougat MIUI 9 ROM.
A senior XDA developer andr68rus has released an un-official Android 8.1 custom ROM for the device. You can download OmniROM 8.1 Beta for Xiaomi Mi Mix 2 using the links below and flash it via TWRP.