How to downgrade Android version ? (Huawei GR5)

Hi,
I installed the last update HUAWEI_GR5_Firmware_KII-L21_Android 6.0.1_EMUI 4.0_C185B310CUSTC185D004
and i want to downgrade to 5.1 firmware ?
i tried to put the old firmware to dload folder on SD card but it doesn't work
any ideas please ? thank you
ps: sorry for bad english
ps2: my device is huawei GR5 KII-L21

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