Xiaomi Chinese handset manufacturer has started rolling out miui 9 to some selected devices starting from xiaomi redmi note 4 and Mi max 2. They already made it official that the MIUI 9 will be made available on OTA. But, you can actually install it manually on your device if the need arises.

>>You need to unlock your bootloader. To do that, power off your phone then hold the volume down button and power volume simultaneously the fastboot should come up.
>>Download Xiaomi Redmi Note 4 Fastboot ROM
>>Download Xiaomi Mi Max 2 Fastboot ROM
>>Downlaod MiFlash tool
>>Open Miflash tools then select clean all in the right down corner
>>Cick Select and browse to a folder where you extracted the Fastboot ROM and click Flash
>>It will start flashing the phone and will take around 3-5 Minutes depending on the PC and device.

Source:http://www.techfashy.com/2017/11/how-to-install-miui9-on-xiaomi-redmi.html?m=1

You did not outline the steps to unlock bootloader. It's not a 'one line' step as you have portrayed it. People will easily brick their phones if they use this guide.