Nintendo just released the 4.0.0 firmware update for the Nintendo Switch this evening. The highlights of the update include the ability to capture a 30-second long clip of the last moments of gameplay. The update also introduces a way to transfer your user and save data from one system to another.

There are a lot of additional pieces in this update including new user icons, the ability to pre-purchase items from the Nintendo eShop, updates to the news feed, and more.
Improvements Included in Version 4.0.0 (Distributed 10/18/2017)
Added the following system functionality
  • Capture video on select games
    • To capture video, hold down the Capture Button during gameplay
    • Up to a maximum of the previous 30 seconds will be saved in the Album. You can trim the beginning and end of each clip, and post to Facebook and Twitter.
    • As of October 18th, 2017, this feature is compatible with The Legend of Zelda: Breath of the Wild, Mario Kart 8 Deluxe, ARMS, and Splatoon 2

  • Select from 12 new Super Mario Odyssey and The Legend of Zelda: Breath of the Wild icons for your user
    • To edit your user icon, head to your My Page on the top left of the Home Menu > Profile

  • Transfer user and save data to another system
  • Pre-purchase option on Nintendo eShop
    • A pre-purchase option will be available for certain games. This option allows pre-load of the game to your system for quicker play when the game is released.
    • This feature will be supported by future game releases

  • News channel updates
    • The news feed has been updated with a new look.
    • Unfollowing a channel will remove that channel's content from the news feed and following the channel again will make it reappear.

  • Match software version with a group of local users
    • To create a group, head to the software's Options > Software Update > Match Version with Local Users
    • Everyone's software will be updated to match the most recent version in the group
    • All users must be on system menu version 4.0.0 or later to view and join a group

General system stability improvements to enhance the user's experience, including:
  • Changed the specification which hid wireless networks using TKIP security from the network search results. Wireless networks using TKIP security will now display in search results as a grayed-out selection instead of not being displayed
    • The Nintendo Switch console supports WEP, WPA-PSK(AES), and WPA2-PSK(AES). If your router is using a different security type (e.g. WPA-PSK(TKIP)), you will need to change this security type within your router's settings.

The update should automatically download for most users. If you don't yet have the update you can manually start the update by going to your System Settings menu and starting it there.

It should also be noted that there is an update specifically for the Joy-Cons. From what I've heard you need to update your system first. Afterwards, go to your device settings and there should be an option in the Controllers section to update them. There doesn't seem to be any sort of public changelog available for the Joy-Con update.