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so long story short im trying to replace the casing on a set of joy cons and the casing that i bought… doesn’t have a instruction manual in the box ._. the rear casing won’t come off after unscrewing all the screws also help???

https://www.youtube.com/watch?v=ymBbKWzL

Hello. I believe that the ifixit guide for joy cons might help. They explain how to take them apart and disassemble it. Just use the part where they talk about joycons Nintendo Switch Teardown

Did you remove the rails before you took out the screws? The back plastic should come off relatively easily. Be sure to open it like a book with the part that slides into the switch as the books spine. Check out this guide for how to get into the case.