Such files are typically distributed by warez groups for use on hacked 3DS consoles. Downloading or sharing them is copyright infringement. Official updates are available through the Nintendo eShop (but would require a Japanese 3DS and legitimate game copy).
Place the update CIA file in a folder (often named /cias/ ) on your 3DS SD card. Install via FBI: Open the FBI Homebrew App on your 3DS. Navigate to SD > cias . Monster Hunter 4G 3DS -JPN- -Update- CIA
A standard Update CIA contains: