Xiaomi Mobile Phone System 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.

Step 1:
Download the latest MIUI ROM file Download here
There is no need to do it again if you've already downloaded the latest ROM file.

Step 2:
Connect your device to the Windows PC/laptop via a micro USB cable, and copy the ROM file downloaded in Step 1 into the folder 'downloaded_rom' in the internal storage of your device.

Step 3:
Launch 'Updater' app on your device：
MIUI 6: Click the '…' icon at the top-right corner, select 'Choose update package', and choose the ROM file you've put in 'downloaded_rom' in Step 2.
MIUI V5: Press the Menu button, select 'Choose update package', and choose the ROM file you've put in 'downloaded_rom' in Step 2.

Step 4:
After choosing the right ROM file, your device will begin upgrading. Your device should automatically boot to the new version when the update is completed.
Source: http://www.xiaomidevice.com/blog/xiaomi-mobile-phone-system-update/