Good luck on your journey through the Kalos region!
For the uninitiated, a .cia file is the standard installable format for games, applications, and updates on the Nintendo 3DS. Think of it as the 3DS equivalent of a .exe or .apk file. While the term "CIA" in this context is not officially used by Nintendo, it has become the universal standard for distributing software to be installed via a homebrew application like FBI on a hacked 3DS, or for loading into an emulator like Citra.
Navigate to the root directory and open or create a folder named cias .
Verify your base game version region (USA/EUR/JPN) and download the corresponding matching update patch package. Outdated Luma3DS system modules or custom themes.