ALL change-log:

Nintendo Switch Online* features and functionality have been added, including:
Save Data Cloud Backup
User your internet connection to back up game save data for compatible games
Some games are not compatible with Save Data Cloud
To back up save data or download a previous backup, head to System Settings > Data Management > Save Data Cloud Backup
*Nintendo Switch Online membership (sold separately) and Nintendo Account required for online play. Not available in all countries. Internet access required for online features. Save Data Cloud backup available in compatible games. Terms apply. To learn more, click here.
Added the following system functionality
Upload up to four Album screenshots at once on supported social network services
Only one captured video can be uploaded at once
Select from six new Captain Toad icons for your user
To edit your user icon, head to your My Page on the top left of the Home Menu > Profile
Play your digital software and content on non-primary consoles by linking your Nintendo Account
The term "active console" has been renamed "primary console" in Nintendo eShops
Playing software in multiple consoles has certain restrictions
Display of Nintendo Switch News articles will be limited to match the restricted software parental controls setting selections
Please note that the restriction will only apply to News articles distributed after the release of version 6.0.0.
Change the layout of the USB keyboard to the desired language
To change the language, head to the System Settings > System > USB Keyboard
Removed the following system functionality
After installing the system update, it will no longer be possible to unlink your Nintendo Account from your Nintendo Switch user
General system stability improvements to enhance the user's experience, including:
Compatibility improvements have been made for a controller licensed by Nintendo