Here is a structured blog-style post regarding that update:
As a minor patch, 1.1.7 was small, requiring minimal block space on a 3DS SD card. How to Get the 1.1.7 Update on 3DS Super Smash Bros- Update 1.1.7 DLC 3DS -EUR U...
Online and local play
This is typically required if you are playing a dumped/backup copy of the game on a modified 3DS (CFW) or an emulator (like Citra), as the official Nintendo eShop is no longer available to download updates directly. Here is a structured blog-style post regarding that