If you attempt to apply a USA Update 1.5 file to a EUR base ROM, the emulator will treat them as two completely different games, and the update will not apply. How to Install Pokémon X Update 1.5 in Citra
If the emulator throws an error stating the file is encrypted, you downloaded an untouched retail patch meant for a real 3DS console. You will need to use a tool like Batch CIA 3DS Decryptor on your PC to convert it into a decrypted file before installing. Game Displays a Black Screen on Boot pokemon x update 15 decrypted 3ds eur usa
In the world of 3DS preservation and emulation, a "decrypted" file is a ROM that has been processed to remove the standard Nintendo 3DS encryption. This step is vital because: If you attempt to apply a USA Update 1