While there was significant anticipation and some unofficial releases for a 7.1.1 update, the primarily runs on Android 6.0 Marshmallow with ColorOS 3.0.
Navigate to and select the downloaded Android 7.1.1 Nougat .zip file.
Download the official OTA zip file over a reliable Wi-Fi network to avoid file corruption.
Do you have access to a if a local SD card upgrade fails?