Copy the update.zip file to the of your internal storage or external SD card. Do not put it inside "Downloads" or "MIUI" folders.
ui_print("Target: SYSTEM partition update"); mount("ext4", "EMMC", "/dev/block/bootdevice/by-name/system", "/system"); package_extract_dir("system", "/system"); set_perm_recursive(0, 0, 0755, 0644, "/system/app"); unmount("/system"); ui_print("System update complete.");