Update 15 is a patch released by Nintendo to update Pokémon X to version 1.5. This update fixes several bugs, improves stability, and adds new features to the game. The update was initially released for the 3DS console, but has since been decrypted and made available for download.
If you have Luma3DS installed, you can use the encrypted version, but here is how to apply it if you only found a decrypted CIA: pokemon x update 15 decrypted 3ds eur usa updated
The process of applying a decrypted update to your game depends entirely on your chosen platform. For PC / Mobile Emulators Open your emulator directory. Update 15 is a patch released by Nintendo