Why not just use the eShop? Because if you are running a setup (forcing out-of-region software), Nintendo’s official eShop will block you. The CIA format allows you to install the update directly to your SD card via FBI or BigBlueMenu, bypassing Nintendo’s region checks entirely.
Remove the SD card and insert it into your computer or mobile device. pokemon x update 15 3ds world cia region f
Disclaimer: Modifying your console and installing unofficial CIA files can violate Nintendo’s terms of service. Ensure you are aware of the risks and are acting within your local laws. Why not just use the eShop