How can i backup n flash ROM n reinstall it and how to Root S6 edge plus T mobile?

I bought Samsung S6 galaxy edge plus T mobile carrier. Bt in every 5 seconds i get notification of security threat. I read on net its due to Securitylogagent or KNOX. Bt as we know these are system apps so we cant disable them.
I want to know how i backup my phone ROM n settings n how to root it? Plz if anyone can guide step by step.
My phone has Marshmallow 6.0.1.

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