downgrade mah-al00 9.0 to 8.0

plz help.................
i want to downgrade 9.0
C:\Program Files (x86)\Minimal ADB and Fastboot>fastboot oem get-product-model
...
(bootloader) MHA-AL00
OKAY [ 0.009s]
finished. total time: 0.011s


C:\Program Files (x86)\Minimal ADB and Fastboot>fastboot getvar vendorcountry
vendorcountry: all/cn
finished. total time: 0.009s


C:\Program Files (x86)\Minimal ADB and Fastboot>
C:\Program Files (x86)\Minimal ADB and Fastboot>
C:\Program Files (x86)\Minimal ADB and Fastboot>fastboot oem get-build-number
...
(bootloader) :MHA-AL00 9.1.0.212(C00E211R1P14)
OKAY [ 0.010s]
finished. total time: 0.012s


C:\Program Files (x86)\Minimal ADB and Fastboot>fastboot getvar rescue_enter_recovery
getvar:rescue_enter_recovery FAILED (remote: FAIL:need all image flashed!)
finished. total time: 0.027s


C:\Program Files (x86)\Minimal ADB and Fastboot>fastboot oem oeminforead-SYSTEM_VERSION
...
(bootloader) :MHA-AL00 9.0.1.179(C00E64R1P14)
OKAY [ 0.013s]
finished. total time: 0.016s