Hi ! Hope you have a great day ! For the .cia, I'll say the basis, and if there is a problem anyway don't hesitate to report the problem in more detail so I can see where the problem can be ! To install the .cia, go to fbi, search in your "/cias" folder, and click on the Deltarune.cia, then install and you're good ! If it install, but don't show up in the menu, it's probably because of the privcode of the .cia, if that's the problem, I can send you a version with a different privcode !
Hope this help, and you'll like the game !