Man its about 3 tb it gonna take too much time 
No. There is no need for backing up your /home/* data, only because changing DE.
The only exception is a general precaution for every system task, in case you make typographic mistakes. There is no other reason.
I follow instructions! But now problem! My PC explodeded. My hand fell off and now I have Windows 8 (without .1 update) again!!!
I admire you guys’ patience for our new friend who doesn’t seem to know even common internet forum etiquette 
So, how did it go? Did you remove ɢɴᴏᴍᴇ? How do you like Plasma?
Dude, just YouTube how to install Plasma, its not complicated. Once you have it installed, then you YouTube how to remove Gnome.
If you get stuck, post here.
Which one of them? 
Anyways, sorry for your loss 
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Both of them!
Don’t worry, I still got muh feet. I can still make beautiful music, like the Gnu song by RS.
Just don’t follow random instructions you find on WWW again! 
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Install SDDM then install kde plasma. You don’t need to delete gnome if you don’t want to as you might want to go back to it at some point. With SDDM you can choose which DE you would like to use. You can check the Arch Wiki to see how to install kde Plasma. Even if you remove gnome you will not loose your personal data just the gnome applications.
I would be cautious of having multiple DEs installed. This might create some strange issues that are difficult to troubleshoot.
In any case, it’s easy to do the same procedure again and switch back to ɢɴᴏᴍᴇ if one desires to (probably not). No need to keep it installed if one is not using it.
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I know, I just suggested keeping both because the OP is new to Kde and might decide that he prefers gnome. I have multiple DE’s installed. It seems the worse that can happen is that each DE gets updated when updating ones system. Do you see any other problems with keeping multiple DE’s besides extra space?