1.First of all make sure you have root
2.Disable any Internet connection: data and wireless
3.Use a file Explorer with root privileges and go to /system/build.prop.
4.Find the line with ro.miui.secure=0 and change it to 1.
5.Update your system via stock recovery or cwm recovery, depending on which you have installed.
6.Still do not enable any Internet connection.
7.Go again to /system/build.prop and change ro.miui.secure=0