How to downgrade

I hawe samsung s6 and just updeted to newest android 7.1.1 and it is a total crap (theme is wery bad dont like it, also it deleted my previus wallpaper and screen lock. And the most anojing thing its become wery slow and lagiy) how can i downgrade to previus version.?! Becos the latest on is basicly more a pece of sh**

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