How to reflash Firmware?

Hello I updated my m3 note to 5.1.12.0G and I tried to make some adjustments: deleted some bloatware. Now I cant find root settings and It says my phone is unrooted, I cant reflash My phone again becouse Its says Firmware corrupted. So, how to reflash it to 5.1.12.0G again? sry for bad English

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