The phone currently has the MIUI 8 Global build which is running on Android Marshmallow. For upgrading to Nougat, users will have to download the ROM from Xiaomi's MIUI website.
Locate Redmi Note 4 on the list of phones and click on it to explore different ROM's available for the various version of the phone. There are three versions of ROMs namely Redmi Note 4 MTK Global, Redmi Note 4 Qualcomm Global and Redmi Note 4 MTK China. For users in India, the Redmi Note 4 Qualcomm is to be selected for downloading.
After clicking, two tabs will appear with options of stable ROM and Developer ROM. Stable ROM is the regular stable build for MIUI 8 which has been rolled out for the Redmi Note 4 previously. Users need to select Developer ROM to upgrade their Redmi Note 4 to MIUI 9 with Android Nougat.
After following the install instructions carefully, which includes wiping all data in recovery mode, download the 1.5 GB update on your PC.
Connect your Redmi Note 4 to the PC and copy the downloaded ROM file to "downloaded_rom” folder in the internal storage of the phone. This folder can be located in the phone using the Explorer app.
After finishing the transfer to the phone and locating the folder, click on the three-dots on the top right corner of your phone's screen and choose update package. The update will start automatically and the phone will boot up to the new version after it is installed.
Xiaomi has provided the MIUI 9 update currently only to the Redmi Note 4 in India, but more devices are expected to follow it.