Mi4 was a flagship device of Chinese Smart Phone Manufacturer Xiaomi, launched in 2014 but hopped into market in early 2015 which is still doing good for Xiaomi as per the numbers after its predecessor Mi3 which was a good performers of 2014 that brought fame for the brand. To quote here, The prices of the device were recently slashed as well.
Even after so long run and though it is a flagship device Xiaomi delayed in updating the operating system of the phone with their MiUi to lollipop. But now all of a sudden they have released the much waited update of Marshmallow 6.0.1. This update is available as On-the-air update as well as a package to install from recovery(recovery is a section of the phone with gives access for the user to install software packages for their phones.
Using On-the-air updater:
This method is simple and requires less data and time to finish the job:
Using Software package:
Download the latest Rom package from this official link http://en.miui.com/download-240.html and open updater from settings and select “Choose update file from local drive” option and select downloaded file to start updating or just use Xiaomi Pc suit to update it. You can download PC suit and details regarding updating using this software from http://en.miui.com/thread-92720-1-1.html .
If the file is not available in normal download section check in the Development ROM section for the latest ROM which is ready to be released from Xiaomi for your device.