Copy the downloaded .cia update file to your 3DS SD card (e.g., into a folder named /cias/ ). Open FBI: Launch the FBI application on your 3DS Home Menu.
This is the open-source file manager and "installer" for the 3DS. It allows you to take a CIA file from your SD card and "inject" it into the system's memory. How to Download 3DS CIA Files Safely download 3ds cia files upd