This page is a general FAQ that should answer most of the common questions about the Nintendo Switch.
You can't at the moment. The Switch is a fairly new device and it will be some time before any vulnerabilities are exploited and released.
To prevent your device from automatically updating, navigate to System Settings > System on your Switch and switch “Auto-Update Software” to “OFF”. If you have an update already downloaded (noted by “System update is ready” under “System Update” in System Settings > System), you can remove it by holding down the power button and selecting “Reboot” from the prompt that pops up. If this doesn't clear the update properly, try rebooting into Maintenance/Recovery Mode: hold Volume + and - while powering on or rebooting.
To prevent updates from automatically downloading, go to System Settings > Internet > Internet Settings > Connected network/SSID here > Change Settings > DNS Settings > Manual and set your Primary/Secondary DNS to 126.96.36.199.
If you'd like to block updates manually, block this domain via DNS, host file, router firewall, or etc: