I want root my Xperia z2 D6502 version 5.1?

Hi guys I am new in the best form where I see in the world thanx to accept me
I wany help guys I want root my Xperia z2 D6502 version 5.1 ? OK how I can do that
If icant do that
Iwant to downgrade my version 5.1 to kitkat if ican how I do that
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