1. Depending on your model, download Huawei P9 Lite Nougat rollback package which will change current software info to C900B300. This is a specific firmware key that allows to fully update the firmware to any other version.
2. Depending on your model, download Huawei P9 Lite full OTA Software file. It will switching back Huawei P9 Lite to Marshmallow OS.
3. Extract rollback package file to get dload folder and place it into microSD card.
4. Put microSD card into Huawei and turn off the device.
5. Now press Volume Up + Volume Down and Power Button all together. The rollback package flashing process will start automatically.
6. After it’s done, your firmware version should now be B300.
7. Erase microSD card and extract OTA Software file. Once again, you’ll get dload folder that must be placed into microSD card.
8. Repeat step 5 to downgrade Huawei P9 Lite from Nougat 7.0 to Marshmallow 6.0
That’s it! Enjoy your downgraded Huawei P9 Lite hopefully without any bugs.