Aqui teneis el tutorial en la misma web esta
http://en.miui.com/a-232.html
Method 1: System Update
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.
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There is no need to do it again if you’ve already downloaded the latest ROM file.
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.
Launch ‘Updater' app on your device：
MIUI 6/7/8：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.
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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.