Xiaomi Mobile Phone Recovery Update

Note:Please wipe all data in Recovery mode if you want to update to a discontinuous ROM version, or downgrade to an older ROM version using MIUI full ROM pack.
This method is not applicable to some Redmi MTK devices due to differences in Recovery interface.

Step 1:
Download the latest MIUI ROM file Download here
Rename the downloaded ROM file to ‘update.zip’ on the computer.

Step 2:
Connect your device to the Windows PC/laptop via a micro USB cable, and copy the ROM file downloaded and renamed in Step 1 into the root directory of the internal storage of your device. (Do not put it in any folder)

Step 3:
Enter the Recovery mode of your device. There are 2 methods to do it as follows:
Method 1: Launch ‘Updater’ app on your device, click the ‘…’ icon at the top-right corner, and select ‘Reboot to Recovery mode’ to enter.
Method 2: You can also turn off your device and then hold both Volume+ button and Power button at the same time to enter Recovery mode.

Step 4:
In Recovery mode, you can use Volume +/- to select up/down, and Power button to confirm.
After entering Recovery mode, choose the language you use, select ‘Install update.zip to System One’ and confirm, and then your device will begin updating automatically. Wait until the update is completed, choose ‘Reboot to System One’, and then your device should boot to the new version.

Source: http://www.xiaomidevice.com/blog/xiaomi-mobile-phone-recovery-update/