If you are seeing an "Update Required" prompt even after installing the CIA, consider the following:
: This update is mandatory for all online functionality, including the Festival Plaza, GTS, and Wonder Trade.
Absolutely not. A stock 3DS can only install official updates signed by Nintendo. The WORLD CIA lacks the correct signature for your region’s eShop, so FBI will not run, and the system will reject it.
For the vast majority of users searching for this term, installation via a CIA file is the goal. This process requires a 3DS with custom firmware (CFW), such as Luma3DS. The following is for informational and educational purposes. You should only install files from trusted sources and always back up your system's NAND and save data before proceeding.