https://www.gamespot.com/articles/monster-hunter-generations-ultimate-how-to-transfe/1100-6461504/

Capcom's popular monster hunting series arrived on Nintendo Switch this week with the worldwide release of Monster Hunter Generations Ultimate, an expanded port of 3DS's Monster Hunter Generations. If you've already sunk a considerable amount of time into the 3DS version and don't want to start over, you can transfer your progress to the Switch game and carry on with the same hunter. Doing so is a fairly straightforward process, but there are quite a few steps you'll need to follow, which is why we've put together this handy guide to help you along.

To transfer your 3DS save data to the new Switch game, you'll first need to install the Monster Hunter Generations Ultimate Save Data Transfer App on 3DS. You can download the app for free from the 3DS Eshop or directly on Nintendo's website. Once that has been installed, launch the Save Data Transfer App, then choose the option to link your 3DS and Switch systems. This will generate a transfer password. Be advised that you'll need to have an internet connection in order to generate a password.