You can use the eShop to install game updates. Alternatively, you can download the CIA file for the update you want and install it with FBI.
Edit: Ignore my first part. I forgot about the internet disconnectivity problem.[/QUOTE
Copy the game update cia file to the root of your sd card.
Launch FBI, go into your sd card directory, then select the game update cia file to install it.
I don't think it would work, since eshop will only have the US updates (since his/her 2ds is US region).
Okay, I copied it to the root of my SD card. I went into the SD directory and found the update CIA, but I don't see an option to install it.
