Launch the game. You should see a version number (such as v1.1.0 ) on the title screen or in the 3DS home menu. Alternative: Updating via hShop or Universal Updater
For users with custom firmware (CFW) like Luma3DS, the most common way to update is by using to install a .cia file.
Even on modded consoles, you can often download official updates directly from the Home Menu if prompted, as long as your system is connected to the internet.