Question: how to install mtz file in miui 9?
Answer: Download the MTZ file of any MIUI file you wish to install.
Download the latest version of Mi Themer (at least v2. 5).
Install it on your phone.
Open the app by tapping on its icon.
Tap “Select . ... 
Browse and locate the MTZ file.
Tap on it to select it.
Tap the “OK” button.