Update 9/13/2018: Nintendo has announced the Switch’s 6.0.0 system update will arrive alongside the Online Service activation on September 18.
The update will be pushed out to users at 5pm PT / 8pm ET (that’s September 19 at 1am UK / 10am AEST). The Nintendo eShop will undergo maintenance for 3 hours after the update is released.
While little is known about what improvements will come with 6.0.0, it will likely be responsible for enabling cloud saving and other new online features.
Switch users have also been informed that, following 6.0.0, Switch User Accounts will be tied permanently to the Nintendo Account they’re currently tied to. Make sure you’ve linked the correct accounts before that date.