When your 3DS is connected to the Internet, open the application, tap settings, and then tap update. It's rare for a Nintendo downloadable title or application to recieve an update, so there's no general notification to see.
When you update, it'll offer to add a pause button to all data and activate AR viewer hidden functions (a note button, background settings, scale settings, and a stopwatch button). The official website notes
Previously, you had to collect a lot of Stickers by scanning Pokémon AR Markers to unlock these features. Now they will be unlocked right away when you run the update!
And no, Pikachu hasn't learned "fart" with the new update.