How can I just swap the outer shells/casing?

Thanks for reading this much!

All I want to do is swap the outer shells or casing or whatever they're called. I have a blue new 3DS XL and I want to swap the colour for a black one. My friend was going to sell hers, but was kind enough to give it to me so I have all the materials I should need in that regard.

Let me make it clear; I do not want to change anything but the very outer casings. I have a vague understanding of how to swap the bottom one, but I am confused on the top one. Every tutorial that I've looked at has had people unscrewing things on the inside of the top, but if all I want to do is change the outer casing, then will I still need to do that? I feel like it should just come off, if I'm not changing the inside.